MSDN Magazine - January 2009 - (Page 22)

Unmanaged sapi.dll SpVoice ( COM Object ) Managed mscorlib.dll RCW System ComObject ConsoleApplication1.exe .NET Client given you a reference to the RCW, and .NET will take care of releasing it for you. But if the method gives you back an IntPtr, it means you have a reference to the COM object itself, and you almost always have to call Marshal.Release on it (this depends on the precise semantics of whatever method gave you that IntPtr). Here’s how: Dim com As IntPtr = Dim rcw = Marshal.GetObjectForIUnknown(com) Marshal.Release(com) TypeLibrary: ITypeInfo Figure 2 Getting a Runtime Callable Wrapper But it is far more common to declare the function with marshaling so that the marshaler calls GetObjectForI Unknown and Release automatically, as in the declaration of CoCreateInstance that you see in Figure 3. When a Type Lacks an RCW What happens if you don’t have an interop assembly for a COM object? For example, what if you created the COM object itself via CoCreateInstance, or if, as often happens, you call a method on a COM object and it returns a COM object whose type isn’t known in advance? What if you’ve written a managed plugin for an un managed application, and the application gave you a COM object? What if you discovered which COM object to create by looking through the registry? Using ITypeInfo A nice trick for understanding how an object works is to inspect it using reflection. Every one of these things will give you an IntPtr reference to the COM object rather than an Object reference to its RCW. When you ask for an RCW around that IntPtr, you get what is illustrated in Figure 2. In Figure 2 you’ll see that the CLR provided a default RCW, an instance of the default interop type “System ComObject”. If you reflect on this like so Dim b = CoCreateInstance(CLSID_WebBrowser, _ Nothing, 1, IID_IUnknown) Console.WriteLine("DUMP {0}", b.GetType()) For Each member In b.GetType().GetMembers() Console.WriteLine(member) Next ITypeInfo is the equivalent of System.Type for COM classes and interfaces. With it you can enumerate the members of a class or interface. In this example, I’m going to print them out; howev er, you could use ITypeInfo to look up members at run time and then invoke them or get their property values through IDispatch. Figure 4 shows how ITypeInfo fits in, as well as all the other struc tures you’ll need to use. The first step is getting the ITypeInfo for a given COM ob ject. It would be nice if you could use rcw.GetType(), but alas this returns the System.Type information about the RCW it self. It would also be nice if you could use the builtin function Marshal.GetITypeInfoForType(rcw), but unfortunately this only works for RCWs that come from interop assemblies. Instead, you will have to get the ITypeInfo manually. The following code will work for both of these cases, whether the RCW came from the stub in mscorlib, or from a proper in terop assembly: Dim idisp = CType(rcw, IDispatch) Dim count As UInteger = 0 idisp.GetTypeInfoCount(count) If count < 1 Then Throw New Exception("No type info") Dim _typeinfo As IntPtr idisp.GetTypeInfo(0, 0, _typeinfo) If _typeinfo = IntPtr.Zero Then Throw New Exception("No ITypeInfo") Dim typeInfo = CType(Marshal.GetTypedObjectForIUnknown(_typeinfo, _ GetType(ComTypes.ITypeInfo)), ComTypes.ITypeInfo) Marshal.Release(_typeinfo) This code uses the interface IDispatch. The interface isn’t defined Figure 3 CoCreateInstance _ Function CoCreateInstance( _ ByRef clsid As Guid, _ ByVal punkOuter As Object, _ ByVal context As Integer, _ ByRef iid As Guid) _ As Object End Function Dim IID_NULL As Guid = New Guid("00000000-0000-0000-C000-000000000000") Dim IID_IUnknown As Guid = New _ Guid("00000000-0000-0000-C000-000000000046") Dim CLSID_SpVoice As Guid = New _ Guid("96749377-3391-11D2-9EE3-00C04F797396") Dim b As Object = CoCreateInstance(CLSID_SpVoice, Nothing, 1, _ IID_IUnknown) you’ll find that it doesn’t have any members that are useful to you; it only has these: DUMP System ComObject System.Object GetLifetimeService() System.Object InitializeLifetimeService() System.Runtime.Remoting.ObjRef CreateObjRef(System.Type) System.String ToString() Boolean Equals(System.Object) Int32 GetHashCode() System.Type GetType() To get useful reflection on such a COM object, you have to reflect on its TypeLibrary yourself. You can do so using ITypeInfo. But first, a brief note: if a method gives you back an Object or IDispatch or ITypeInfo or other .NET class or interface, then it has 22 msdn magazine Basic instincts

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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.