发信人: zillion (心诚则灵), 信区: DotNET
标 题: Andy's DOTNET FAQ
发信站: BBS 水木清华站 (Sun May 6 20:25:47 2001)
http://www.andymcm.com/dotnetfaq.htm
Contents
1. Introduction
1.1 What is .NET?
1.2 Is .NET just a new name for Windows DNA?
1.3 Does .NET only apply to people building web-sites?
1.4 When was .NET announced?
1.5 When will .NET be released?
1.6 How do I develop .NET apps?
1.7 Where can I download the .NET SDK & Visual Studio 7?
1.8 What are the key technologies within .NET?
1.9 What platforms will the .NET Framework run on?
1.10 What languages will the .NET Framework support?
1.11 Will the .NET Framework go through a standardisation process?
2. Basic terminology
2.1 What is the CLR?
2.2 What is the CTS?
2.3 What is the CLS?
2.4 What is IL?
2.5 What is C#?
2.6 What does 'managed' mean in the .NET context?
2.7 What is reflection?
3. Assemblies
3.1 What is an assembly?
3.2 How can I produce an assembly?
3.3 What is the difference between a private assembly and a shared
assembly?
3.4 How do assemblies find each other?
3.5 How does assembly versioning work?
4. Application Domains
4.1 What is an Application Domain?
4.2 How does an AppDomain get created?
4.3 Can I write my own .NET host?
5. Garbage Collection
5.1 What is garbage collection?
5.2 Is it true that objects don't always get destroyed immediately
when the last reference goes away?
5.3 Why doesn't the .NET runtime offer deterministic destruction?
5.4 Is the lack of deterministic destruction in .NET a problem?
5.5 Does non-deterministic destruction affect the usage of COM objects
from managed code?
5.6 I've heard that Finalize methods should be avoided. Should I
implement Finalize on my class?
5.7 Do I have any control over the garbage collection algorithm?
5.8 How can I find out what the garbage collector is doing?
6. Attributes
6.1 What are attributes?
6.2 Can I create my own metadata attributes?
6.3 Can I create my own context attributes?
7. Code Access Security
7.1 What is Code Access Security (CAS)?
7.2 How does CAS work?
7.3 Who defines the CAS code groups?
7.4 How do I define my own code group?
7.5 How do I change the permission set for a code group?
7.6 Can I create my own permission set?
7.7 I'm having some trouble with CAS. How can I diagnose my problem?
7.8 I can't be bothered with all this CAS stuff. Can I turn it off?
8. Intermediate Language (IL)
8.1 Can I look at the IL for an assembly?
8.2 Can source code be reverse-engineered from IL?
8.3 How can I stop my code being reverse-engineered from IL?
8.4 Can I write IL programs directly?
8.5 Can I do things in IL that I can't do in C#?
9. Implications for COM
9.1 Is COM dead?
9.2 Is DCOM dead?
9.3 Is MTS/COM+ dead?
9.4 Can I use COM components from .NET programs?
9.5 Can I use .NET components from COM programs?
9.6 Is ATL redundant in the .NET world?
10. Miscellaneous
10.1 How does .NET remoting work?
10.2 How can I get at the Win32 API from a .NET program?
11. Class Library
11.1 File I/O
11.1.1 How do I read from a text file?
11.1.2 How do I write to a text file?
11.1.3 How do I read/write binary files?
11.1.4 How do I delete a file?
11.2 Text Processing
11.2.1 Are regular expressions supported?
11.3 Internet
11.3.1 How do I download a web page?
11.3.2 How do I use a proxy?
11.4 XML
11.4.1 Is DOM supported?
11.4.2 Is SAX supported?
11.4.3 Is XPath supported?
11.5 Threading
11.5.1 Is multi-threading supported?
11.5.2 How do I spawn a thread?
11.5.3 How do I stop a thread?
11.5.4 How do I use the thread pool?
11.5.5 How do I know when my thread pool work item has completed?
11.5.6 How do I prevent concurrent access to my data?
11.6 Tracing
11.6.1 Is there built-in support for tracing/logging?
11.6.2 Can I redirect tracing to a file?
11.6.3 Can I customise the trace output?
12. Resources
12.1 Where can I find out more about .NET?
12.2 Sample code & utilities
1. Introduction
1.1 What is .NET?
That's difficult to sum up in a sentence. According to Microsoft, .NET
is a "revolutionary new platform, built on open Internet protocols and
standards, with tools and services that meld computing and
communications in new ways".
A more practical definition would be that .NET is a new environment
for developing and running software applications, featuring ease of
development of web-based services, rich standard run-time services
available to components written in a variety of programming languages,
and inter-language and inter-machine interoperability.
Note that when the term ".NET" is used in this FAQ it refers only to the
new .NET runtime and associated technologies. This is sometimes
called the ".NET Framework". This FAQ does NOT cover any of the
various other existing and new products/technologies that Microsoft
are attaching the .NET name to (e.g. SQL Server.NET).
1.2 Is .NET just a new name for Windows DNA?
No. In many ways, the term Windows DNA was just a marketing term for
an approach (i.e. the 3-tier approach) to using existing technology. .
NET is much more radical and includes a complete new software
development and runtime framework.
1.3 Does .NET only apply to people building web-sites?
No. If you write any Windows software (using ATL/COM, MFC, VB, or even
raw Win32), .NET may offer a viable alternative (or addition) to the way
you do things currently. Of course, if you do develop web sites, then
.NET has lots to interest you - not least ASP+.
1.4 When was .NET announced?
Bill Gates delivered a keynote at Forum 2000, held June 22, 2000,
outlining the .NET 'vision'. The July 2000 PDC had a number of
sessions on .NET technology, and delegates were given CDs containing a
pre-release version of the .NET framework/SDK and Visual Studio 7.
1.5 When will .NET be released?
No firm date yet, but it is expected in the second half of 2001.
1.6 How do I develop .NET apps?
The .NET Framework SDK contains command-line compilers and utilities
which can be used to build .NET apps. The next version of Visual
Studio (called Visual Studio 7 or Visual Studio.NET) will have fully
integrated support for .NET development.
1.7 Where can I download the .NET SDK & Visual Studio 7?
You can download Beta 1 of the SDK from http://msdn.microsoft.com/net/.
If you are an MSDN Universal subscriber, you can also download Beta 1
of Visual Studio 7.
1.8 What are the key technologies within .NET?
ASP.NET, CLR (Common Language Runtime), C# (a new Java-like language),
SOAP, XML, ADO.NET, multi-language support (Eiffel, COBOL etc)
1.9 What platforms will the .NET Framework run on?
Beta 1 supports Windows 2000, NT4 SP6a, Windows Me and Windows 98 for
development. The runtime is supported on Windows 95.
Microsoft are due to release a new version of Windows in a similar
timeframe to the .NET runtime. It is codenamed 'Whistler' and is largely
an incremental update to Windows 2000, with an emphasis on GUI changes.
Microsoft will market this new OS as '.NET-enabled', but apparently
it is not tied to the release of the .NET runtime. If the .NET runtime
is ready in time it will be included with Whistler. If the .NET
runtime is not ready, Whistler will ship without it.
1.10 What languages will the .NET Framework support?
Initially MS will provide compilers for C#, C++, VB and JScript. Other
vendors have announced that they intend to develop .NET compilers for
languages such as COBOL, Eiffel, Perl, Smalltalk and Python.
1.11 Will the .NET Framework go through a standardisation process?
Proposed draft standards for C# and something called the 'Common
Language Infrastructure' have been submitted to ECMA. See http://msdn.
microsoft.com/net/ecma/
2. Basic terminology
2.1 What is the CLR?
CLR = Common Language Runtime. The CLR is a set of standard resources
that (in theory) any .NET program can take advantage of, regardless of
programming language. Robert Schmidt (Microsoft) lists the following CLR
resources in his MSDN PDC# article:
Object-oriented programming model (inheritance, polymorphism,
exception handling, garbage collection)
Security model
Type system
All .NET base classes
Many .NET framework classes
Development, debugging, and profiling tools
Execution and code management
IL-to-native translators and optimizers
What this means is that in the .NET world, different programming
languages will be more equal in capability than they have ever been
before, although clearly not all languages will support all CLR
services.
2.2 What is the CTS?
CTS = Common Type System. This is the range of types that the .NET
runtime understands, and therefore that .NET applications can use.
However note that not all .NET languages will support all the types in
the CTS. The CTS is a superset of the CLS.
2.3 What is the CLS?
CLS = Common Language Specification. This is a subset of the CTS which
all .NET languages are expected to support. The idea is that any program
which uses CLS-compliant types can interoperate with any .NET program
written in any language.
In theory this allows very tight interop between different .NET
languages - for example allowing a C# class to inherit from a VB class.
2.4 What is IL?
IL = Intermediate Language. Also known as MSIL. All .NET source code (of
any language) is compiled to IL. The IL is then converted to machine
code at the point where the software is installed, or at run-time by a
Just-In-Time (JIT) compiler.
2.5 What is C#?
C# is a new language which will run inside the .NET framework. In
their "Introduction to C#" whitepaper, Microsoft describe C# as
follows:
"C# is a simple, modern, object oriented, and type-safe programming
language derived from C and C++. C# (pronounced sharp? is firmly
planted in the C and C++ family tree of languages, and will
immediately be familiar to C and C++ programmers. C# aims to combine the
high productivity of Visual Basic and the raw power of C++."
Substitute 'Java' for 'C#' in the quote above, and you'll see that the
statement still works pretty well :-).
If you are a C++ programmer, you might like to check out my C# FAQ.
2.6 What does 'managed' mean in the .NET context?
The term 'managed' is the cause of much confusion. It is used in various
places within .NET, meaning slightly different things.
Managed code: The .NET framework provides several core run-time services
to the programs that run within it - for example exception handling and
security. For these services to work, the code must provide a minimum
level of information to the runtime. Such code is called managed code.
All C#, Visual Basic.NET and JScript.NET code is managed by default. VS7
C++ code is not managed by default, but the compiler can produce
managed code by specifying a command-line switch (/com+).
Managed data: This is data that is allocated and de-allocated by the .
NET runtime's garbage collector. C#, VB.NET and JScript.NET data is
always managed. VS7 C++ data is unmanaged by default, even when using
the /com+ switch, but it can be marked as managed using the __gc
keyword.
Managed classes: This is usually referred to in the context of Managed
Extensions (ME) for C++. When using ME C++, a class can be marked with
the __gc keyword. As the name suggests, this means that the memory for
instances of the class is managed by the garbage collector, but it
also means more than that. The class becomes a fully paid-up member of
the .NET community with the benefits and restrictions that brings. An
example of a benefit is proper interop with classes written in other
languages - for example, a managed C++ class can inherit from a VB
class. An example of a restriction is that a managed class can only
inherit from one base class.
2.7 What is reflection?
All .NET compilers produce metadata about the types defined in the
modules they produce. This metadata is packaged along with the module
(modules in turn are packaged together in assemblies), and can be
accessed by a mechanism called reflection. The System.Reflection
namespace contains classes that can be used to interrogate the types for
a module/assembly.
Using reflection to access .NET metadata is very similar to using
ITypeLib/ITypeInfo to access type library data in COM, and it is used
for similar purposes - e.g. determining data type sizes for marshaling
data across context/process/machine boundaries.
Reflection can also be used to dynamically invoke methods (see System.
Type.InvokeMember), or even create types dynamically at run-time (see
System.Reflection.Emit.TypeBuilder).
3. Assemblies
3.1 What is an assembly?
An assembly is sometimes described as a logical .EXE or .DLL, and can be
an application (with a main entry point) or a library. An assembly
consists of one or more files (dlls, exes, html files etc), and
represents a group of resources, type definitions, and implementations
of those types. An assembly may also contain references to other
assemblies. These resources, types and references are described in a
block of data called a manifest. The manifest is part of the assembly,
thus making the assembly self-describing.
An important aspect of assemblies is that they are part of the
identity of a type. The identity of a type is the assembly that houses
it combined with the type name. This means, for example, that if
assembly A exports a type called T, and assembly B exports a type called
T, the .NET runtime sees these as two completely different types.
Furthermore, don't get confused between assemblies and namespaces -
namespaces are merely a hierarchical way of organising type names. To
the runtime, type names are type names, regardless of whether namespaces
are used to organise the names. It's the assembly plus the typename
(regardless of whether the type name belongs to a namespace) that
uniquely indentifies a type to the runtime.
Assemblies are also important in .NET with respect to security - many of
the security restrictions are enforced at the assembly boundary.
Finally, assemblies are the unit of versioning in .NET - more on this
below.
3.2 How can I produce an assembly?
The simplest way to produce an assembly is directly from a .NET
compiler. For example, the following C# program:
public class CTest
{
public CTest()
{
System.Console.WriteLine( "Hello from CTest" );
}
}
can be compiled into a library assembly (dll) like this:
csc /t:library ctest.cs
You can then view the contents of the assembly by running the "IL
Disassembler" tool that comes with the .NET SDK.
Alternatively you can compile your source into modules, and then combine
the modules into an assembly using the assembly linker (al.exe). For
the C# compiler, the /target:module switch is used to generate a
module instead of an assembly.
3.3 What is the difference between a private assembly and a shared
assembly?
Location and visibility: A private assembly is normally used by a single
application, and is stored in the application's directory, or a
sub-directory beneath. A shared assembly is normally stored in the
global assembly cache, which is a repository of assemblies maintained by
the .NET runtime. Shared assemblies are usually libraries of code which
many applications will find useful, e.g. the .NET framework classes.
Versioning: The runtime enforces versioning constraints only on shared
assemblies, not on private assemblies.
3.4 How do assemblies find each other?
By searching directory paths. There are several factors which can affect
the path (such as the AppDomain host, and application configuration
files), but for private assemblies the search path is normally the
application's directory and its sub-directories. For shared assemblies,
the search path is normally same as the private assembly path plus
the shared assembly cache.
3.5 How does assembly versioning work?
Each assembly has a version number called the compatibility version.
Also each reference to an assembly (from another assembly) includes both
the name and version of the referenced assembly.
The version number has four numeric parts (e.g. 5.5.2.33). Assemblies
with either of the first two parts different are normally viewed as
incompatible. If the first two parts are the same, but the third is
different, the assemblies are deemed as 'maybe compatible'. If only
the fourth part is different, the assemblies are deemed compatible.
However, this is just the default guideline - it is the version policy
that decides to what extent these rules are enforced. The version policy
can be specified via the application configuration file.
Remember: versioning is only applied to shared assemblies, not private
assemblies.
4. Application Domains
4.1 What is an Application Domain?
An AppDomain can be thought of as a lightweight process. Multiple
AppDomains can exist inside a Win32 process. The primary purpose of
the AppDomain is to isolate an application from other applications.
Win32 processes provide isolation by having distinct memory address
spaces. This is effective, but it is expensive and doesn't scale well.
The .NET runtime enforces AppDomain isolation by keeping control over
the use of memory - all memory in the AppDomain is managed by the .NET
runtime, so the runtime can ensure that AppDomains do not access each
other's memory.
4.2 How does an AppDomain get created?
AppDomains are usually created by hosts. Examples of hosts are the
Windows Shell, ASP+ and IE. When you run a .NET application from the
command-line, the host is the Shell. The Shell creates a new AppDomain
for every application.
AppDomains can also be explicitly created by .NET applications. Here
is a C# sample which creates an AppDomain, creates an instance of an
object inside it, and then executes one of the object's methods:
using System;
using System.Runtime.Remoting;
public class CAppDomainInfo : MarshalByRefObject
{
public string GetAppDomainInfo()
{
return "AppDomain = " + AppDomain.CurrentDomain.FriendlyName;
}
}
public class App
{
public static int Main()
{
AppDomain ad = AppDomain.CreateDomain( "Andy's new domain", null, null
);
ObjectHandle oh = ad.CreateInstance( "appdomaintest.exe",
"CAppDomainInfo" );
CAppDomainInfo adInfo = (CAppDomainInfo)(oh.Unwrap());
string info = adInfo.GetAppDomainInfo();
Console.WriteLine( "AppDomain info: " + info );
return 0;
}
}
4.3 Can I write my own .NET host?
Yes. For an example of how to do this, take a look at the source for the
dm.net moniker developed by Jason Whittington and Don Box (http:
//staff.develop.com/jasonw/clr/readme.htm). There is also a code
sample in the .NET SDK called CorHost.
5. Garbage Collection
5.1 What is garbage collection?
Garbage collection is a system whereby a run-time component takes
responsibility for managing the lifetime of objects and the heap
memory that they occupy. This concept is not new to .NET - Java and many
other languages/runtimes have used garbage collection for some time.
5.2 Is it true that objects don't always get destroyed immediately
when the last reference goes away?
Yes. The garbage collector offers no guarantees about the time when an
object will be destroyed and its memory reclaimed.
There is an interesting thread in the archives, started by Chris Sells,
about the implications of non-deterministic destruction of objects in
C#: http://discuss.develop.com/archives/wa.
exe?A2=ind0007&L=DOTNET&P=R24819
In October 2000, Microsoft's Brian Harry posted a lengthy analysis of
the problem: http://discuss.develop.com/archives/wa.
exe?A2=ind0010A&L=DOTNET&P=R28572
Chris Sells' response to Brian's posting is here: http://discuss.
develop.com/archives/wa.exe?A2=ind0010C&L=DOTNET&P=R983
5.3 Why doesn't the .NET runtime offer deterministic destruction?
Because of the garbage collection algorithm. The .NET garbage
collector works by periodically running through a list of all the
objects that are currently being referenced by an application. All the
objects that it doesn't find during this search are ready to be
destroyed and the memory reclaimed. The implication of this algorithm is
that the runtime doesn't get notified immediately when the final
reference on an object goes away - it only finds out during the next
sweep of the heap.
Futhermore, this type of algorithm works best by performing the
garbage collection sweep as rarely as possible. Normally heap exhaustion
is the trigger for a collection sweep.
5.4 Is the lack of deterministic destruction in .NET a problem?
It's certainly an issue that affects component design. If you have
objects that maintain expensive or scarce resources (e.g. database
locks), you need to provide some way for the client to tell the object
to release the resource when it is done. Microsoft recommend that you
provide a method called Dispose() for this purpose. However, this causes
problems for distributed objects - in a distributed system who calls
the Dispose() method? Some form of reference-counting or
ownership-management mechanism is needed to handle distributed objects -
unfortunately the runtime offers no help with this.
5.5 Does non-deterministic destruction affect the usage of COM objects
from managed code?
Yes. When using a COM object from managed code, you are effectively
relying on the garbage collector to call the final release on your
object. If your COM object holds onto an expensive resource which is
only cleaned-up after the final release, you may need to provide a new
interface on your object which supports an explicit Dispose() method.
5.6 I've heard that Finalize methods should be avoided. Should I
implement Finalize on my class?
An object with a Finalize method is more work for the garbage
collector than an object without one. Also there are no guarantees about
the order in which objects are Finalized, so there are issues
surrounding access to other objects from the Finalize method. Finally,
there is no guarantee that a Finalize method will get called on an
object, so it should never be relied upon to do clean-up of an
object's resources.
Microsoft recommend the following pattern:
public class CTest
{
public override void Dispose()
{
... // Cleanup activities
GC.SuppressFinalize(this);
}
protected override void Finalize()
{
Dispose();
}
}
In the normal case the client calls Dispose(), the object's resources
are freed, and the garbage collector is relieved of its Finalizing
duties by the call to SuppressFinalize(). In the worst case, i.e. the
client forgets to call Dispose(), there is a reasonable chance that
the object's resources will eventually get freed by the garbage
collector calling Finalize(). Given the limitations of the garbage
collection algorithm this seems like a pretty reasonable approach.
5.7 Do I have any control over the garbage collection algorithm?
A little. The System.GC class exposes a couple of interesting methods.
The first is the Collect method - this forces the garbage collector to
collect all unreferenced objects immediately. The other is
RequestFinalizeOnShutdown(), which tells the garbage collector to
definitely run the Finalize() method of each object on shutdown of the
application. Normally the garbage collector puts a speedy exit at a
higher priority than calling Finalize during application shutdown, so
this method could be handy to force the runtime to be a little more
responsible.
If you want to verify that this is not just a theoretical issue, try the
following test program:
using System;
class CTest
{
protected override void Finalize()
{
Console.WriteLine( "This is the Finalizer." );
}
}
class CApplication
{
public static void Main()
{
Console.WriteLine( "This is Main." );
CTest test = new CTest();
// GC.RequestFinalizeOnShutdown();
}
}
Run the program, then re-run with the GC.RequestFinalizeOnShutdown()
line un-commented. You should notice a difference ...
5.8 How can I find out what the garbage collector is doing?
Lots of interesting statistics are exported from the .NET runtime via
the 'COM+ Memory' performance object. Use Performance Monitor to view
them.
6. Attributes
6.1 What are attributes?
There are at least two types of .NET attribute. The first type I will
refer to as a metadata attribute - it allows some data to be attached to
a class or method. This data becomes part of the metadata for the
class, and (like other class metadata) can be accessed via reflection.
An example of a metadata attribute is [serializable], which can be
attached to a class and means that instances of the class can be
serialized.
[serializable] public class CTest {}
The other type of attribute is a context attribute. Context attributes
use a similar syntax to metadata attributes but they are fundamentally
different. Context attributes provide an interception mechanism
whereby instance activation and method calls can be pre- and/or
post-processed. If you've come across Keith Brown's universal
delegator you'll be familiar with this idea.
6.2 Can I create my own metadata attributes?
Yes. Simply derive a class from System.Attribute and mark it with the
AttributeUsage attribute. For example:
[AttributeUsage(AttributeTargets.Class)]
public class InspiredByAttribute : System.Attribute
{
public string InspiredBy;
public InspiredByAttribute( string inspiredBy )
{
InspiredBy = inspiredBy;
}
}
[InspiredBy("Andy Mc's brilliant .NET FAQ")]
class CTest
{
}
class CApp
{
public static void Main()
{
object[] atts = typeof(CTest).GetCustomAttributes();
foreach( object att in atts )
if( att is InspiredByAttribute )
Console.WriteLine( "Class CTest was inspired by {0}",
((InspiredByAttribute)att).InspiredBy );
}
}
6.3 Can I create my own context attributes?
Yes. Take a look at Don Box's sample (called CallThreshold) at http:
//www.develop.com/dbox/dotnet/threshold/, and also Peter Drayton's
Tracehook.NET at http://www.razorsoft.net/
7. Code Access Security
7.1 What is Code Access Security (CAS)?
CAS is the part of the .NET security model that determines whether or
not a piece of code is allowed to run, and what resources it can use
when it is running. For example, it is CAS that will prevent a .NET
web applet from formatting your hard disk.
7.2 How does CAS work?
The CAS security policy revolves around two key concepts - code groups
and permissions. Each .NET assembly is a member of a particular code
group, and each code group is granted the permissions specified in a
named permission set.
For example, using the default security policy, a control downloaded
from a web site belongs to the 'Zone - Internet' code group, which
adheres to the permissions defined by the 'Internet' named permission
set. (Naturally the 'Internet' named permission set represents a very
restrictive range of permissions.)
7.3 Who defines the CAS code groups?
Microsoft defines some default ones, but you can modify these and even
create your own. To see the code groups defined on your system, run
'caspol -lg' from the command-line. On my system it looks like this:
Level = Machine
Code Groups:
1. All code: Nothing
1.1. Zone - MyComputer: FullTrust
1.1.1. Honor SkipVerification requests: SkipVerification
1.2. Zone - Intranet: LocalIntranet
1.3. Zone - Internet: Internet
1.4. Zone - Untrusted: Nothing
1.5. Zone - Trusted: Internet
1.6. StrongName -
0024000004800000940000000602000000240000525341310004000003
000000CFCB3291AA715FE99D40D49040336F9056D7886FED46775BC7BB5430BA4444FEF8
348EBD06
F962F39776AE4DC3B7B04A7FE6F49F25F740423EBF2C0B89698D8D08AC48D69CED0FC8F8
3B465E08
07AC11EC1DCC7D054E807A43336DDE408A5393A48556123272CEEEE72F1660B71927D385
61AABF5C
AC1DF1734633C602F8F2D5: Everything
Note the hierarchy of code groups - the top of the hierarchy is the most
general ('All code'), which is then sub-divided into several groups,
each of which in turn can be sub-divided. Also note that (somewhat
counter-intuitively) a sub-group can be associated with a more
permissive permission set than its parent.
7.4 How do I define my own code group?
Use caspol. For example, suppose you trust code from www.mydomain.com
and you want it have full access to your system, but you want to keep
the default restrictions for all other internet sites. To achieve this,
you would add a new code group as a sub-group of the 'Zone -
Internet' group, like this:
caspol -ag 1.3 -site www.mydomain.com FullTrust
Now if you run caspol -lg you will see that the new group has been added
as group 1.3.1:
...
1.3. Zone - Internet: Internet
1.3.1. Site - www.mydomain.com: FullTrust
...
Note that the numeric label (1.3.1) is just a caspol invention to make
the code groups easy to manipulate from the command-line. The underlying
runtime never sees it.
7.5 How do I change the permission set for a code group?
Use caspol. If you are the machine administrator, you can operate at the
'machine' level - which means not only that the changes you make become
the default for the machine, but also that users cannot change the
permissions to be more permissive. If you are a normal (non-admin)
user you can still modify the permissions, but only to make them more
restrictive. For example, to allow intranet code to do what it likes you
might do this:
caspol -cg 1.2 FullTrust
Note that because this is more permissive than the default policy (on
a standard system), you should only do this at the machine level - doing
it at the user level will have no effect.
7.6 Can I create my own permission set?
Yes. Use caspol -ap, specifying an XML file containing the permissions
in the permission set. To save you some time, here is a sample file
corresponding to the 'Everything' permission set - just edit to suit
your needs. When you have edited the sample, add it to the range of
available permission sets like this:
caspol -ap samplepermset.xml
Then, to apply the permission set to a code group, do something like
this:
caspol -cg 1.3 SamplePermSet
(By default, 1.3 is the 'Internet' code group)
7.7 I'm having some trouble with CAS. How can I diagnose my problem?
Caspol has a couple of options that might help. First, you can ask
caspol to tell you what code group an assembly belongs to, using
caspol -rsg. Similarly, you can ask what permissions are being applied
to a particular assembly using caspol -rsp.
7.8 I can't be bothered with all this CAS stuff. Can I turn it off?
Yes, as long as you are an administrator. Just run:
caspol -s off
8. Intermediate Language (IL)
8.1 Can I look at the IL for an assembly?
Yes. MS supply a tool called Ildasm which can be used to view the
metadata and IL for an assembly.
8.2 Can source code be reverse-engineered from IL?
Yes, it is often relatively straightforward to regenerate high-level
source (e.g. C#) from IL.
8.3 How can I stop my code being reverse-engineered from IL?
The only method currently available is to run ilasm with the /owner
option. The IL for the resulting assembly cannot be viewed with ildasm.
However, a determined code-thief could hack ildasm or write their own
version of ildasm, so this method is only useful for deterring the
casual thief.
Unfortunately the current .NET compilers do not have a /owner option, so
to protect your C# or VB.NET assembly you'll need to do something
like this:
csc helloworld.cs
ildasm /out=temp.il helloworld.exe
ilasm /owner temp.il
(This suggestion was posted to the DOTNET list by Hany Ramadan.)
In the longer term it is likely that IL obfuscation tools will become
available (either from MS or from third parties). These tools work by
'optimising' the IL in such a way that reverse-engineering becomes
much more difficult.
Of course if you are writing web services then reverse-engineering is
not a problem as clients do not have access to your IL.
8.4 Can I write IL programs directly?
Yes. Peter Drayton posted this simple example to the DOTNET mailing
list:
.assembly MyAssembly {}
.class MyApp {
.method static void Main() {
.entrypoint
ldstr "Hello, IL!"
call void System.Console::WriteLine(class System.Object)
ret
}
}
Just put this into a file called hello.il, and then run ilasm hello.il.
An exe assembly will be generated.
8.5 Can I do things in IL that I can't do in C#?
Yes. A couple of simple examples are that you can throw exceptions
that are not derived from System.Exception, and you can have
non-zero-based arrays.
9. Implications for COM
9.1 Is COM dead?
This subject causes a lot of controversy, as you'll see if you read
the mailing list archives. Take a look at the following two threads:
http://discuss.develop.com/archives/wa.
exe?A2=ind0007&L=DOTNET&D=0&P=68241
http://discuss.develop.com/archives/wa.
exe?A2=ind0007&L=DOTNET&P=R60761
FWIW my view is as follows: COM is many things, and it's different
things to different people. But to me, COM is fundamentally about how
little blobs of code find other little blobs of code, and how they
communicate with each other when they find each other. COM specifies
precisely how this location and communication takes place. In a 'pure'
.NET world, consisting entirely of .NET objects, little blobs of code
still find each other and talk to each other, but they don't use COM
to do so. They use a model which is similar to COM in some ways - for
example, type information is stored in a tabular form packaged with
the component, which is quite similar to packaging a type library with a
COM component. But it's not COM.
So, does this matter? Well, I don't really care about most of the COM
stuff going away - I don't care that finding components doesn't
involve a trip to the registry, or that I don't use IDL to define my
interfaces. But there is one thing that I wouldn't like to go away - I
wouldn't like to lose the idea of interface-based development. COM's
greatest strength, in my opinion, is its insistence on a cast-iron
separation between interface and implementation. Unfortunately, the .NET
framework seems to make no such insistence - it lets you do
interface-based development, but it doesn't insist. Some people would
argue that having a choice can never be a bad thing, and maybe they're
right, but I can't help feeling that maybe it's a backward step.
9.2 Is DCOM dead?
Pretty much, for .NET developers. The .NET Framework has a new
remoting model which is not based on DCOM. Of course DCOM will still
be used in interop scenarios.
9.3 Is MTS/COM+ dead?
No. The approach for the first .NET release is to provide access to
the existing COM+ services (through an interop layer) rather than
replace the services with native .NET ones. Various tools and attributes
are provided to try to make this as painless as possible. The PDC
release of the .NET SDK includes interop support for core services
(JIT activation, transactions) but not some of the higher level services
(e.g. COM+ Events, Queued components).
Over time it is expected that interop will become more seamless - this
may mean that some services become a core part of the CLR, and/or it may
mean that some services will be rewritten as managed code which runs on
top of the CLR.
For more on this topic, search for postings by Joe Long in the
archives - Joe is the MS group manager for COM+. Start with this
message:
http://discuss.develop.com/archives/wa.
exe?A2=ind0007&L=DOTNET&P=R68370
9.4 Can I use COM components from .NET programs?
Yes. COM components are accessed from the .NET runtime via a Runtime
Callable Wrapper (RCW). This wrapper turns the COM interfaces exposed by
the COM component into .NET-compatible interfaces. For oleautomation
interfaces, the RCW can be generated automatically from a type library.
For non-oleautomation interfaces, it may be necessary to develop a
custom RCW which manually maps the types exposed by the COM interface to
.NET-compatible types.
Here's a simple example for those familiar with ATL. First, create an
ATL component which implements the following IDL:
import "oaidl.idl";
import "ocidl.idl";
[
object,
uuid(EA013F93-487A-4403-86EC-FD9FEE5E6206),
helpstring("ICppName Interface"),
pointer_default(unique),
oleautomation
]
interface ICppName : IUnknown
{
[helpstring("method SetName")] HRESULT SetName([in] BSTR name);
[helpstring("method GetName")] HRESULT GetName([out,retval] BSTR *pName
);
};
[
uuid(F5E4C61D-D93A-4295-A4B4-2453D4A4484D),
version(1.0),
helpstring("cppcomserver 1.0 Type Library")
]
library CPPCOMSERVERLib
{
importlib("stdole32.tlb");
importlib("stdole2.tlb");
[
uuid(600CE6D9-5ED7-4B4D-BB49-E8D5D5096F70),
helpstring("CppName Class")
]
coclass CppName
{
[default] interface ICppName;
};
};
When you've built the component, you should get a typelibrary. Run the
TLBIMP utility on the typelibary, like this:
tlbimp cppcomserver.tlb
If successful, you will get a message like this:
Typelib imported successfully to CPPCOMSERVERLib.dll
You now need a .NET client - let's use C#. Create a .cs file
containing the following code:
using System;
using CPPCOMSERVERLib;
public class MainApp
{
static public void Main()
{
CppName cppname = new CppName();
cppname.SetName( "bob" );
Console.WriteLine( "Name is " + cppname.GetName() );
}
}
Note that we are using the type library name as a namespace, and the COM
class name as the class. Alternatively we could have used
CPPCOMSERVERLib.CppName for the class name and gone without the using
CPPCOMSERVERLib statement.
Compile the C# code like this:
csc /r:cppcomserverlib.dll csharpcomclient.cs
Note that the compiler is being told to reference the DLL we
previously generated from the typelibrary using TLBIMP.
You should now be able to run csharpcomclient.exe, and get the following
output on the console:
Name is bob
9.5 Can I use .NET components from COM programs?
Yes. .NET components are accessed from COM via a COM Callable Wrapper
(CCW). This is similar to a RCW (see previous question), but works in
the opposite direction. Again, if the wrapper cannot be automatically
generated by the .NET development tools, or if the automatic behaviour
is not desirable, a custom CCW can be developed. Also, for COM to
'see' the .NET component, the .NET component must be registered in the
registry.
Here's a simple example. Create a C# file called testcomserver.cs and
put the following in it:
using System;
namespace AndyMc
{
public class CSharpCOMServer
{
public CSharpCOMServer() {}
public void SetName( string name ) { m_name = name; }
public string GetName() { return m_name; }
private string m_name;
}
}
Then compile the .cs file as follows:
csc /target:library testcomserver.cs
You should get a dll, which you register like this:
regasm testcomserver.dll /tlb:testcomserver.tlb
Now you need to create a client to test your .NET COM component.
VBScript will do - put the following in a file called comclient.vbs:
Dim dotNetObj
Set dotNetObj = CreateObject("AndyMc.CSharpCOMServer")
dotNetObj.SetName ("bob")
MsgBox "Name is " & dotNetObj.GetName()
and run the script like this:
wscript comclient.vbs
And hey presto you should get a message box displayed with the text
"Name is bob".
(Note that at the time of writing there seem to be some path issues with
accessing .NET classes as COM components - to avoid problems, run
comclient.vbs from the same directory as testcomserver.dll)
An alternative to the approach above it to use the dm.net moniker
developed by Jason Whittington and Don Box. Go to http://staff.develop.
com/jasonw/clr/readme.htm to check it out.
9.6 Is ATL redundant in the .NET world?
Yes, if you are writing applications that live inside the .NET
framework. Of course many developers may wish to continue using ATL to
write C++ COM components that live outside the framework, but if you are
inside you will almost certainly want to use C#. Raw C++ (and therefore
ATL which is based on it) doesn't have much of a place in the .NET
world - it's just too near the metal and provides too much flexibility
for the runtime to be able to manage it.
10. Miscellaneous
10.1 How does .NET remoting work?
.NET remoting involves sending messages along channels. Two of the
standard channels are HTTP and TCP. TCP is intended for LANs only - HTTP
can be used for LANs or WANs (internet).
Support is provided for multiple message serializarion formats. Examples
are SOAP (XML-based) and binary. By default, the HTTP channel uses SOAP
(via the .NET runtime Serialization SOAP Formatter), and the TCP
channel uses binary (via the .NET runtime Serialization Binary
Formatter). But either channel can use either serialization format.
There are a number of styles of remote access:
SingleCall. Each incoming request from a client is serviced by a new
object. The object is thrown away when the request has finished. This
(essentially stateless) model can be made stateful in the ASP+
environment by using the ASP+ state service to store application or
session state.
Singleton. All incoming requests from clients are processed by a
single server object.
Client-activated object. This is the old stateful (D)COM model whereby
the client receives a reference to the remote object and holds that
reference (thus keeping the remote object alive) until it is finished
with it.
Distributed garbage collection of objects is managed by a system
called 'leased based lifetime'. Each object has a lease time, and when
that time expires the object is disconnected from the .NET runtime
remoting infrastructure. Objects have a default renew time - the lease
is renewed when a successful call is made from the client to the object.
The client can also explicitly renew the lease.
If you're interested in using XML-RPC as an alternative to SOAP, take
a look at Charles Cook's XML-RPC.Net site at http://www.cookcomputing.
com/xmlrpc/xmlrpc.shtml.
10.2 How can I get at the Win32 API from a .NET program?
Use P/Invoke. This uses similar technology to COM Interop, but is used
to access static DLL entry points instead of COM objects. Here is an
example of C# calling the Win32 MessageBox function:
using System;
using System.Runtime.InteropServices;
class MainApp
{
[dllimport("user32.dll", EntryPoint="MessageBox", SetLastError=true,
CharSet=CharSet.Auto)]
public static extern int MessageBox(int hWnd, String strMessage, String
strCaption, uint uiType);
public static void Main()
{
MessageBox( 0, "Hello, this is PInvoke in operation!", ".NET", 0 );
}
}
11. Class Library
11.1 File I/O
11.1.1 How do I read from a text file?
First, use a System.IO.FileStream object to open the file:
FileStream fs = new FileStream( @"c:\test.txt",FileMode.Open,
FileAccess.Read );
FileStream inherits from Stream, so you can wrap the FileStream object
with a StreamReader object. This provides a nice interface for
processing the stream line by line:
StreamReader sr = new StreamReader( fs );
string curLine;
while( (curLine = sr.ReadLine()) != null )
Console.WriteLine( curLine );
Finally close the StreamReader object:
sr.Close();
Note that this will automatically call Close() on the underlying
Stream object, so an explicit fs.Close() is not required.
11.1.2 How do I write to a text file?
Similar to the read example, except use StreamWriter instead of
StreamReader.
11.1.3 How do I read/write binary files?
Similar to text files, except wrap the FileStream object with a
BinaryReader/Writer object instead of a StreamReader/Writer object.
11.1.4 How do I delete a file?
Use the static Delete() method on the System.IO.File object:
File.Delete( @"c:\test.txt");
11.2 Text Processing
11.2.1 Are regular expressions supported?
Yes. Use the System.Text.RegularExpressions.Regex class. For example,
the following code updates the title in an HTML file:
FileStream fs = new FileStream( "test.htm", FileMode.Open, FileAccess.
Read );
StreamReader sr = new StreamReader( fs );
Regex r = new Regex( "<TITLE>(.*)</TITLE>" );
string s;
while( (s = sr.ReadLine()) != null )
{
if( r.IsMatch( s ) )
s = r.Replace( s, "<TITLE>New and improved ${1}</TITLE>" );
Console.WriteLine( s );
}
11.3 Internet
11.3.1 How do I download a web page?
First use the System.Net.WebRequestFactory class to acquire a WebRequest
object:
WebRequest request = WebRequestFactory.Create( "http://localhost");
Then ask for the response from the request:
WebResponse response = request.GetResponse();
The GetResponse method blocks until the download is complete. Then you
can access the response stream like this:
Stream s = response.GetResponseStream();
// Output the downloaded stream to the console
StreamReader sr = new StreamReader( s );
string line;
while( (line = sr.ReadLine()) != null )
Console.WriteLine( line );
Note that WebRequest and WebReponse objects can be downcast to
HttpWebRequest and HttpWebReponse objects respectively, to access
http-specific functionality.
11.3.2 How do I use a proxy?
Two approaches - to affect all web requests do this:
System.Net.GlobalProxySelection.Select = new DefaultControlObject(
"proxyname", 80 );
Alternatively, to set the proxy for a specific web request, do this:
ProxyData proxyData = new ProxyData();
proxyData.HostName = "proxyname";
proxyData.Port = 80;
proxyData.OverrideSelectProxy = true;
HttpWebRequest request = (HttpWebRequest)WebRequestFactory.Create(
"http://localhost");
request.Proxy = proxyData;
11.4 XML
11.4.1 Is DOM supported?
Yes. Take this example XML document:
<PEOPLE>
<PERSON>Fred</PERSON>
<PERSON>Bill</PERSON>
</PEOPLE>
This document can be parsed as follows:
XmlDocument doc = new XmlDocument();
doc.Load( "test.xml" );
XmlNode root = doc.DocumentElement;
foreach( XmlNode personElement in root.ChildNodes )
Console.WriteLine( personElement.FirstChild.Value.ToString() );
The output is:
Fred
Bill
11.4.2 Is SAX supported?
No. Instead, a new XmlReader/XmlWriter API is offered. Like SAX it is
stream-based but it uses a 'pull' model rather than SAX's 'push' model.
Here's an example:
XmlTextReader reader = new XmlTextReader( "test.xml" );
while( reader.Read() )
{
if( reader.NodeType == XmlNodeType.Element && reader.Name == "PERSON"
)
{
reader.Read(); // Skip to the child text
Console.WriteLine( reader.Value );
}
}
11.4.3 Is XPath supported?
Yes, via the XmlNavigator class (DocumentNavigator derives from
XmlNavigator):
XmlDocument doc = new XmlDocument();
doc.Load( "test.xml" );
DocumentNavigator nav = new DocumentNavigator(doc);
nav.MoveToDocument();
nav.Select( "descendant::PEOPLE/PERSON" );
while( nav.MoveToNextSelected() )
{
nav.MoveToFirstChild();
Console.WriteLine( "{0}", nav.Value );
}
11.5 Threading
11.5.1 Is multi-threading supported?
Yes, there is extensive support for multi-threading. New threads can
be spawned, and there is a system-provided threadpool which applications
can use.
11.5.2 How do I spawn a thread?
Create an instance of a System.Threading.Thread object, passing it an
instance of a ThreadStart delegate that will be executed on the new
thread. For example:
class MyThread
{
public MyThread( string initData )
{
m_data = initData;
m_thread = new Thread( new ThreadStart(ThreadMain) );
m_thread.Start();
}
// ThreadMain() is executed on the new thread.
private void ThreadMain()
{
Console.WriteLine( m_data );
}
public void WaitUntilFinished()
{
m_thread.Join();
}
private Thread m_thread;
private string m_data;
}
In this case creating an instance of the MyThread class is sufficient to
spawn the thread and execute the MyThread.ThreadMain() method:
MyThread t = new MyThread( "Hello, world." );
t.WaitUntilFinished();
11.5.3 How do I stop a thread?
There are several options. First, you can use your own communication
mechanism to tell the ThreadStart method to finish. Alternatively the
Thread class has in-built support for instructing the thread to stop.
The two principle methods are Thread.Interrupt() and Thread.Abort(). The
former will cause a ThreadInterruptedException to be thrown on the
thread when it next goes into a WaitJoinSleep state. In other words,
Thread.Interrupt is a polite way of asking the thread to stop when it is
no longer doing any useful work. In contrast, Thread.Abort() throws a
ThreadAbortException regardless of what the thread is doing.
Furthermore, the ThreadAbortException cannot normally be caught
(though the ThreadStart's finally method will be executed). Thread.
Abort() is a heavy-handed mechanism which should not normally be
required.
11.5.4 How do I use the thread pool?
By passing an instance of a WaitCallback delegate to the ThreadPool.
QueueUserWorkItem() method:
class CApp
{
static void Main()
{
string s = "Hello, World";
ThreadPool.QueueUserWorkItem( new WaitCallback( DoWork ), s );
Thread.Sleep( 1000 ); // Give time for work item to be executed
}
// DoWork is executed on a thread from the thread pool.
static void DoWork( object state )
{
Console.WriteLine( state );
}
}
11.5.5 How do I know when my thread pool work item has completed?
There is no way to query the thread pool for this information. You
must put code into the WaitCallback method to signal that it has
completed. Events are useful for this.
11.5.6 How do I prevent concurrent access to my data?
Each object has a concurrency lock (critical section) associated with
it. The System.Threading.Monitor.Enter/Exit methods are used to
acquire and release this lock. For example, instances of the following
class only allow one thread at a time to enter method f():
class C
{
public void f()
{
try
{
Monitor.Enter(this);
...
}
finally
{
Monitor.Exit(this);
}
}
}
C# has a 'lock' keyword which provides a convenient shorthand for the
code above:
class C
{
public void f()
{
lock(this)
{
...
}
}
}
Note that calling Monitor.Enter(myObject) does NOT mean that all
access to myObject is serialized. It means that the synchronisation lock
associated with myObject has been acquired, and no other thread can
acquire that lock until Monitor.Exit(o) is called. In other words,
this class is functionally equivalent to the classes above:
class C
{
public void f()
{
lock( m_object )
{
...
}
}
private m_object = new object();
}
11.6 Tracing
11.6.1 Is there built-in support for tracing/logging?
Yes, in the System.Diagnostics namespace. There are two main classes
that deal with tracing - Debug and Trace. They both work in a similar
way - the difference is that tracing from the Debug class only works
in builds that have the DEBUG symbol defined, whereas tracing from the
Trace class only works in builds that have the TRACE symbol defined.
Typically this means that you should use System.Diagnostics.Trace.
WriteLine for tracing that you want to work in debug and release builds,
and System.Diagnostics.Debug.WriteLine for tracing that you want to
work only in debug builds.
11.6.2 Can I redirect tracing to a file?
Yes. The Debug and Trace classes both have a Listeners property, which
is a collection of sinks that receive the tracing that you send via
Debug.WriteLine and Trace.WriteLine respectively. By default the
Listeners collection contains a single sink, which is an instance of the
DefaultTraceListener class. This sends output to the Win32
OutputDebugString() function and also the System.Diagnostics.Debugger.
Log() method. This is useful when debugging, but if you're trying to
trace a problem at a customer site, redirecting the output to a file
is more appropriate. Fortunately, the TextWriterTraceListener class is
provided for this purpose.
Here's how to use the TextWriterTraceListener class to redirect Trace
output to a file:
Trace.Listeners.Clear();
FileStream fs = new FileStream( @"c:\log.txt",FileMode.Create,
FileAccess.Write );
Trace.Listeners.Add( new TextWriterTraceListener( fs ) );
Trace.WriteLine( @"This will be writen to c:\log.txt!" );
Note the use of Trace.Listeners.Clear() to remove the default listener.
If you don't do this, the output will go to the file and
OutputDebugString(). Typically this is not what you want, because
OutputDebugString() imposes a big performance hit.
11.6.3 Can I customise the trace output?
Yes. You can write your own TraceListener-derived class, and direct
all output through it. Here's a simple example, which derives from
TextWriterTraceListener (and therefore has in-built support for
writing to files, as shown above) and adds timing information and the
thread ID for each trace line:
class MyListener : TextWriterTraceListener
{
public MyListener( Stream s ) : base(s)
{
}
public override void WriteLine( string s )
{
Writer.WriteLine( "{0:D8} [{1:D4}] {2}",
Environment.TickCount - m_startTickCount,
AppDomain.GetCurrentThreadId(),
s );
}
protected int m_startTickCount = Environment.TickCount;
}
(Note that this implementation is not complete - the TraceListener.Write
method is not overridden for example.)
The beauty of this approach is that when an instance of MyListener is
added to the Trace.Listeners collection, all calls to Trace.
WriteLine() go through MyListener, including calls made by referenced
assemblies that know nothing about the MyListener class.
12. Resources
12.1 Where can I find out more about .NET?
The Microsoft .NET homepage is at http://www.microsoft.com/net/.
Microsoft also host GOTDOTNET.
Microsoft also now host a .NET Framework FAQ, quite similar to this one.
Check it out for more 'authoritative' answers to many of the
questions posed here.
Robert Scoble has compiled an extremely comprehensive list of on-line
resources at http://www.devx.com/dotnet/resources/,and there is another
list at http://www.singularidad.com.ar/dotnet.asp
Robert also has a .NET "Famous Questions and Answers" page at http:
//www.devx.com/free/press/2000/vs-qalist.asp
Richard Grimes and Richard Anderson have a site called Managed World.
COM.
www.ibuyspy.com is a sample site created to showcase the .NET platform.
There is also my C# FAQ for C++ Programmers.
12.2 Sample code & utilities
Peter Drayton's .NET Goodies page is at http://www.razorsoft.net/
Don Box's CallThreshold sample is at http://www.develop.
com/dbox/dotnet/threshold
Don's UnwindScope Service is at http://www.develop.
com/dbox/dotnet/unwind
Don's CLR scripting host is at http://www.develop.
com/dbox/dotnet/clrscript
Don & Jason's dm.net COM moniker at http://staff.develop.
com/jasonw/clr/readme.htm
Mike Woodring has some .NET samples at http://www.bearcanyon.
com/dotnet/
Charles Cook's XML-RPC.Net library is available at http://www.
cookcomputing.com/xmlrpc/xmlrpc.shtml.
--
Since there's no help,
Come let us kiss and part.
※ 来源:·BBS 水木清华站 smth.org·[FROM: 211.100.73.91]
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