MSDN Magazine - January 2009 - (Page 26)

such as Integer and String are represented by classes or structures through System.Type; second, in .NET the compound types such as Array of Integer are represented through System.Type. The code that you need to dig through a TYPEDESC is quite straightforward (see Figure 8). Note that the case VT_USERDE FINED again uses a handle to a reference, which you must look up through GetRefTypeInfo. Figure 9 Using VARIANT ''' ''' VARIANT: this is called "Object" in Visual Basic. It's the universal ''' variable type for COM. ''' ''' The "vt" flag determines which of the other fields have ''' meaning. vt is a VarEnum. _ Public Structure [VARIANT] Public vt As UShort _ Public wReserved1 As UShort _ Public wReserved2 As UShort _ Public wReserved3 As UShort ' Public llVal As Long Public lVal As Integer Public bVal As Byte ' and similarly for many other accessors _ Public ptr As System.IntPtr ''' ''' GetObject: returns a .NET Object equivalent for this Variant. ''' Function GetObject() As Object ' We want to use the handy Marshal.GetObjectForNativeVariant. ' But this only operates upon an IntPtr to a block of memory. ' So we first flatten ourselves into that block of memory. (size 16) Dim ptr = Marshal.AllocCoTaskMem(16) Marshal.StructureToPtr(Me, ptr, False) Try : Return Marshal.GetObjectForNativeVariant(ptr) Finally : Marshal.FreeCoTaskMem(ptr) : End Try End Function End Structure The COM Representation of Value The next step is to actually dump a COM object, that is, print out the values of its properties. This task is easy if you know the names of those properties since you can just use a latebound call in Visual Basic: Dim com as Object : Dim val = com.SomePropName The compiler translates this into a runtime call of IDispatch::Invoke to fetch the value of the property. But in the case of reflection, you may not know the property name. Perhaps all you have is MEMBERID, so you have to call IDispatch::Invoke yourself. This isn’t very pretty. The first headache comes from the fact that COM and .NET represent values in very different ways. In .NET you use Object to represent arbitrary values. In COM you use the VARIANT struc ture, as shown in Figure 9. A COM value uses the vt field to indicate what type it is. It might be VarEnum.VT_INT, or VarEnum.VT_PTR, or any of the 30 or so VarEnum types. Knowing its type, you can figure out which of the other fields to look up in a giant Select Case statement. Luck ily, that Select Case statement has already been implemented in the Marshal.GetObjectForNativeVariant function. Dumping the Properties of a COM Object You’ll want to dump the properties of your COM object, more or less like the “Quick Watch” window in Visual Studio: DUMP OF COM OBJECT #28114988 ISpeechVoice.Status = System ComObject As Ref ISpeechVoiceStatus ISpeechVoice.Rate = 0 As Integer ISpeechVoice.Volume = 100 As Integer ISpeechVoice.AllowAudioOutputFormatChangesOnNextSet = True As Bool ISpeechVoice.EventInterests = 0 As SpeechVoiceEvents ISpeechVoice.Priority = 0 As SpeechVoicePriority ISpeechVoice.AlertBoundary = 32 As SpeechVoiceEvents ISpeechVoice.SynchronousSpeakTimeout = 10000 As Integer Figure 10 Looking at Get* and Is* Methods ' We'll only try to retrieve things that are likely to be side-effect' free properties: If (funcDesc.invkind And ComTypes.INVOKEKIND.INVOKE_PROPERTYGET) = 0 _ AndAlso Not funcName Like "[Gg]et*" _ AndAlso Not funcName Like "[Ii]s*" _ Then Continue For If funcDesc.cParams > 0 Then Continue For Dim returnType = CType(funcDesc.elemdescFunc.tdesc.vt, VarEnum) If returnType = VarEnum.VT_VOID Then Continue For Dim returnTypeName = DumpTypeDesc(funcDesc.elemdescFunc.tdesc, typeInfo) ' And we'll only try to evaluate the easily-evaluatable properties: Dim dumpableTypes = New VarEnum() {VarEnum.VT_BOOL, VarEnum.VT_BSTR, _ VarEnum.VT_CLSID, _ VarEnum.VT_DECIMAL, VarEnum.VT_FILETIME, VarEnum.VT_HRESULT, _ VarEnum.VT_I1, VarEnum.VT_I2, VarEnum.VT_I4, VarEnum.VT_I8, _ VarEnum.VT_INT, VarEnum.VT_LPSTR, VarEnum.VT_LPWSTR, _ VarEnum.VT_R4, VarEnum.VT_R8, _ VarEnum.VT_UI1, VarEnum.VT_UI2, VarEnum.VT_UI4, VarEnum.VT_UI8, _ VarEnum.VT_UINT, VarEnum.VT_DATE, _ VarEnum.VT_USERDEFINED} Dim typeIsDumpable = dumpableTypes.Contains(returnType) If returnType = VarEnum.VT_PTR Then Dim ptrType = CType(Marshal.PtrToStructure( _ funcDesc.elemdescFunc.tdesc.lpValue, _ GetType(ComTypes.TYPEDESC)), ComTypes.TYPEDESC) If ptrType.vt = VarEnum.VT_USERDEFINED Then typeIsDumpable = True End If The thing is, there are lots of different types in COM. It would be exhausting to write code to handle every single case correctly and hard to assemble enough test cases to test them all. Here I’ll settle for dumping a small set of types that I know I can handle correctly. And beyond that, what would be useful to dump? Besides the properties, it would also be useful to get a dump of anything ex posed through pure (sideeffectfree) functions such as IsTall(). But you wouldn’t want to invoke functions such as AddRef(). To discriminate between these two, I reckon that any function name such as “Is*” is fair game for dumping (see Figure 10). It turns out that COM programmers seem to use Is* functions a lot less fre quently than do programmers using .NET! In this code, the final kind of dumpable type you consider is a VT_PTR to a VT_USERDEFINED type. This covers the common case of a property that returns a reference to another object. The final step is to read the property you’ve identified by its MEMBERID or invoke the function. You can see the code to do it in Figure 11. The key method here is IDispatch.Invoke. Its first argument is the member id of the property or function that you’re invoking. The variable dispatchType is either 2 for a propertyget or Basic instincts Using IDispatch.Invoke 26 msdn magazine

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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