MSDN Magazine - January 2009 - (Page 24)

Figure 7 Reflecting on Function Members For iFunc = 0 To typeAttr.cFuncs - 1 ' retrieve FUNCDESC: Dim pFuncDesc As IntPtr : typeInfo.GetFuncDesc(iFunc, pFuncDesc) Dim funcDesc = CType(Marshal.PtrToStructure(pFuncDesc, _ GetType(ComTypes.FUNCDESC)), ComTypes.FUNCDESC) Dim names As String() = {""} typeInfo.GetNames(funcDesc.memid, names, 1, 0) Dim funcName = names(0) ' Function formal parameters: Dim cParams = funcDesc.cParams Dim s = "" For iParam = 0 To cParams - 1 Dim elemDesc = CType(Marshal.PtrToStructure( _ New IntPtr(funcDesc.lprgelemdescParam.ToInt64 + _ Marshal.SizeOf(GetType(ComTypes.ELEMDESC)) * iParam), _ GetType(ComTypes.ELEMDESC)), ComTypes.ELEMDESC) If s.Length > 0 Then s &= ", " If (elemDesc.desc.paramdesc.wParamFlags And _ Runtime.InteropServices.ComTypes.PARAMFLAG.PARAMFLAG_FOUT) _ 0 Then s &= "out " s &= DumpTypeDesc(elemDesc.tdesc, typeInfo) Next ' And print out the rest of the function's information: Dim props = "" If (funcDesc.invkind And ComTypes.INVOKEKIND.INVOKE_PROPERTYGET) _ 0 Then props &= "Get " If (funcDesc.invkind And ComTypes.INVOKEKIND.INVOKE_PROPERTYPUT) _ 0 Then props &= "Set " If (funcDesc.invkind And ComTypes.INVOKEKIND.INVOKE_PROPERTYPUTREF) _ 0 Then props &= "Set " Dim isSub = (FUNCDESC.elemdescFunc.tdesc.vt = VarEnum.VT_VOID) s = props & If(isSub, "Sub ", "Function ") & funcName & "(" & s & ")" s &= If(isSub, "", " as " & _ DumpTypeDesc(funcDesc.elemdescFunc.tdesc, typeInfo)) Console.WriteLine(" " & s) typeInfo.ReleaseFuncDesc(pFuncDesc) Next Chasing Down Type References The first task that must be completed is to get a list of imple mented/inherited interfaces. (In COM, one never inherits from another class). Here’s the code: ' Inheritance: For iImplType = 0 To typeAttr.cImplTypes - 1 Dim href As Integer typeInfo.GetRefTypeOfImplType(iImplType, href) ' "href" is an index into the list of type descriptions within the ' type library. Dim implTypeInfo As ComTypes.ITypeInfo typeInfo.GetRefTypeInfo(href, implTypeInfo) ' And GetRefTypeInfo looks up the index to get an ITypeInfo for it. Dim implTypeName = "" implTypeInfo.GetDocumentation(-1, implTypeName, "", 0, "") Console.WriteLine(" Implements {0}", implTypeName) Next There’s a layer of indirection here. GetRefTypeOfImplType doesn’t give you the ITypeInfo of the implemented types directly: instead, it gives you a handle to an ITypeInfo. The function GetRefTy peInfo is what looks up that handle. Then you can use the familiar GetDocumentation(1) to get the name of that implemented type. I’ll discuss handles to ITypeInfo again later. Getting the Members For reflecting on field members, each field has a VARDESC to describe it. Once again, the typeInfo object allocates an unman aged memory block pVarDesc, and you marshal it into a managed block varDesc and release the unmanaged block: ' Field members: For iVar = 0 To typeAttr.cVars - 1 Dim pVarDesc As IntPtr : typeInfo.GetVarDesc(iVar, pVarDesc) Dim varDesc = CType(Marshal.PtrToStructure(pVarDesc, _ GetType(ComTypes.VARDESC)), ComTypes.VARDESC) typeInfo.ReleaseVarDesc(pVarDesc) Dim names As String() = {""} typeInfo.GetNames(varDesc.memid, names, 1, 0) Dim varName = names(0) Console.WriteLine(" Dim {0} As {1}", varName, _ DumpTypeDesc(varDesc.elemdescVar.tdesc, typeInfo)) Next Figure 8 Looking at TYPEDESC Function DumpTypeDesc(ByVal tdesc As ComTypes.TYPEDESC, _ ByVal context As ComTypes.ITypeInfo) As String Dim vt = CType(tdesc.vt, VarEnum) Select Case vt Case VarEnum.VT_PTR Dim tdesc2 = CType(Marshal.PtrToStructure(tdesc.lpValue, _ GetType(ComTypes.TYPEDESC)), ComTypes.TYPEDESC) Return "Ref " & DumpTypeDesc(tdesc2, context) Case VarEnum.VT_USERDEFINED Dim href = CType(tdesc.lpValue.ToInt64 And Integer.MaxValue, Integer) Dim refTypeInfo As ComTypes.ITypeInfo = Nothing context.GetRefTypeInfo(href, refTypeInfo) Dim refTypeName = "" refTypeInfo.GetDocumentation(-1, refTypeName, "", 0, "") Return refTypeName Case VarEnum.VT_CARRAY Dim tdesc2 = CType(Marshal.PtrToStructure(tdesc.lpValue, _ GetType(ComTypes.TYPEDESC)), ComTypes.TYPEDESC) Return "Array of " & DumpTypeDesc(tdesc2, context) ' lpValue is actually an ARRAYDESC structure, which also has ' information on the array dimensions, but alas .NET doesn't ' predefine ARRAYDESC. Case Else ' There are many other VT_s that I haven't special-cased, ' e.g. VT_INTEGER. Return vt.ToString() End Select End Function The function “GetNames” is curious. Conceivably each member might have several names. It’s enough just to get the first. The code for reflecting on function members is generally similar (see Figure 7). The return type is funcDesc.elemdescFunc.tdesc. The count of formal parameters is given by funcDesc.cParams, and formal parameters are stored in the array funcDesc.lprgelemdesc Param. (It’s not pleasant accessing an unmanaged array like this from managed code because you have to do pointer arithmetic.) There are other flags as well as PARAMFLAG_FOUT—flags for in, retval, optional, and so on. Type information for both fields and members was stored in a TYPEDESC structure, and I invoked a function DumpTypeDesc to print it out. It might seem surprising that TYPEDESC is used instead of ITypeInfo. I’ll elaborate next. Primitive and Synthetic Types COM uses TYPEDESC to describe some types and ITypeInfo to describe others. What’s the difference? COM uses ITypeInfo only for classes and interfaces defined in type libraries. And it uses TYPEDESC for primitive types such as Integer or String, and also for compound types such as Array of SpVoice or IUnknown Reference. This is different from .NET: first, in .NET even the primitive types Basic instincts 24 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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