MSDN Magazine - January 2009 - (Page 21)

Lucian Wischik Basic instincts Inspecting COM Objects With Reflection Many of you have felt the frustration of trying to get COM to work. You’ve also felt the joy that comes when you’re suc cessful. A common trick for understanding how an object works is to inspect it using the reflection capabilities of the Microsoft .NET Framework. In some cases, .NET reflec tion also works on COM objects. Look at the following code to see what I mean. The code uses .NET reflection to obtain and display a list of members in the object Dim b As New SpeechLib.SpVoice Console.WriteLine("GETTYPE {0}", b.GetType()) For Each member In b.GetType().GetMembers() Console.WriteLine(member) Next Unmanaged sapi.dll SpVoice ( COM Object ) Managed Interop.SpeechLib.dll RCW SpeechLib.SpVoice ConsoleApplication1.exe .NET Client TypeLibrary: ITypeInfo and produces this output to the console: GETTYPE SpeechLib.SpVoiceClass Void Speak(System.String, UInt32, UInt32 ByRef) Void SetVoice(SpeechLib.ISpObjectToken) Void GetVoice(SpeechLib.ISpObjectToken ByRef) Int32 Volume Figure 1 Reflecting on SpeechLib But this code doesn’t work for all COM objects. For that, you have to use COM reflection. This column explains why and how. Why might you want to use reflection on an object? I’ve found reflection to be useful for debugging and logging; you can use it to write a generalpurpose “dump” routine that prints out everything it can about an object. The code in this column will be enough for you to write your own “dump” routine. Once it’s done, you could even invoke it from the Immediate window while debugging. That’s especially useful since the Visual Studio debugger doesn’t always provide much information about COM objects. For production use, reflection is useful if you’ve written an ap plication that takes plugin components where users drop their components into a directory or list them in the registry, and your application has to examine these components and find which classes and methods they expose. For instance, Visual Studio uses reflection in this way to populate IntelliSense. Let’s build a project for illustration. First, create the project and add a COM reference via Project>AddReference. For this column I’ll use the “Microsoft Speech Object Library” SpeechLib. Figure 1 shows the relevant entities and files that are examined when you run the reflection code you saw earlier. Sapi.dll is the DLL that contains SpeechLib. It happens to live in %windir%\system32\speech\common\sapi.dll. This DLL con tains both the implementation of the SpVoice COM class and a TypeLibrary that contains all reflection information for it. Type Libraries are optional, but almost every COM component on your system will have one. Interop.SpeechLib.dll was automatically generated by Visual Stu dio through Project>AddReference. The generator reflects upon the TypeLibrary and produces an Interop Type for SpVoice. This type is a managed class that contains a managed method for every native COM method found in the TypeLibrary. You can also gen erate interop assemblies yourself using the tlbimp.exe command line tool from the Windows SDK. An instance of an interop type is called a runtime callable wrapper (RCW), and it wraps a pointer to an instance of a COM class. Running the following Visual Basic command creates an RCW (an instance of the interop type) and also an instance of the SpVoice COM class: Dim b As New SpeechLib.SpVoice Type Libraries and Runtime Callable Wrappers The variable “b” references the RCW, so when the code reflected upon “b” it was really reflecting upon the managed equivalent that had been constructed from the TypeLibrary. Users who deploy their ConsoleApplication1.exe will also have to deploy Interop.SpeechLib.dll. (However, Visual Studio 2010 will allow the interop type to be copied directly inside ConsoleApplica tion1.exe. This will greatly simplify deployment. The feature is called “NoPrimaryInteropAssembly,” or “NoPIA” for short.) Send your questions and comments to instinct@microsoft.com. Code download at code.msdn.microsoft.com/mag200901Basic. January 2009 21 http://code.msdn.microsoft.com/mag200901Basic

Table of Contents Feed for the Digital Edition of MSDN Magazine - January 2009

Toolbox CLR Inside Out Basic Instincts Cutting Edge Test Run First Look� Geneva Framework Silverlight Windows Mobile Service Station Security Briefs Extreme ASP.NET Foundations .NET Matters { End Bracket }

MSDN Magazine - January 2009

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