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    <p>COM's way of passing callbacks is interfaces. Trying to match unmanaged function pointers with .NET delegates is complicated and error-prone, as you've experienced. Interfaces are not as practical as delegates, but still better than function pointers.</p> <p>So if I were you I'd put the callbacks in a COM interface exported by the COM DLL:</p> <p>(The following is IDL code, which must go in the .idl file associated with the C++ project.)</p> <pre><code>interface ISomeObject : IUnknown { HRESULT DoTask1([in] int i); HRESULT DoTask2([in] BSTR s); } </code></pre> <p>Then build the C++ project, and add the type library as a reference from the C# project. If the type library is registered, you can add it by right-clicking the C# project name in the <strong>Solution Explorer</strong> pane of Visual Studio, select <strong>Add Reference</strong>, go to the <strong>COM</strong> tab, look for the name of the type library and add it as a reference.</p> <p>Once you have added a reference to the type library, you can use the COM interface as if it were a C# interface:</p> <pre><code>class MyForm : AnotherForm, ISomeObject { // ISomeObject methods: public void DoTask1(int i) { ... } public void DoTask2(string s) { ... } ... } </code></pre> <p>Then InitDotNet would take a ISomeObject pointer, and the C# code would simply call it by passing <strong>this</strong>:</p> <p>C++:</p> <pre><code>ISomeObject* g_pSomeObject; extern "C" __declspec(dllexport) void __stdcall InitDotNet(ISomeObject* o) { g_pSomeObject = o; } </code></pre> <p>C#:</p> <pre><code>[DllImport("mycppdll.dll")] private static extern void InitDotNet(ISomeObject o); private void DoInitDotNet() { // The following works because MyForm implements ISomeObject InitDotNet(this); } </code></pre> <p>But I would also make InitDotNet a method of a COM interface and not a global function.</p> <p>Last but not least, what's the purpose of the VB class? If it's only purpose is only to wrap the COM class, you don't need it: COM classes/interfaces are directly consumable from C#.</p>
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    1. POExported function in C++ COM DLL not found from C#
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    1. COI'm wondering if you are assuming that my DLL is a C# dll. It is an older DLL written in c++. From searching around, this method was all I could find in general approach. And yes, the VB class is just a wrapper because the assumption was that there was no other way to access the COM method in the C++ DLL. You can see above the C++ code - can you suggest an architecture that allows me to call the "mainCall" method passing callbacks in the way you suggest above? Perhaps a sample?
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      1. USIan Frisbie
    2. CONo, I know the DLL is a C++ COM DLL. But are you saying that it's a third-party DLL and you can't modify it? Because to do what I suggested you definitely need to modify it.
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    3. COI can modify it. I would need some explanation though on how the c++ code would interpret the interface pointer.
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