All,

I'm authoring a C++ .dll & C# .dll and need a little help.  I am
experienced in interop but this one has me stumped.  Does anyone have a
simple example of how to declare a C++ exported function that can return
a struct array in one of the parameters to C#?  The C# .dll would have
no idea how many elements would be returned so the C++ .dll would need
to dynamically allocate the array and populate it.  I fully expect to be
told that I'm doing things wrong but I'm just looking for advice on how
to do this correctly.

Here's what I have now.

[C++ .h]
struct MyStruct {
    DWORD dwA;
    double dA;
    double dB;
    DWORD dwB;
    DWORD dwC;
};

extern "C" __declspec(dllexport) HRESULT GetStructArray(MyStruct* structs);

[C++ .cpp]
extern "C" __declspec(dllexport) HRESULT GetStructArray(MyStruct* structs)
{
    structs = (MyStruct)CoTaskMemAlloc(sizeof(MyStruct) * 4);
    /* structs gets populated */
}

[C# .cs]
[StructLayout(LayoutKind.Sequential)]
public struct MyStruct
{
    public uint dwA;
    public double dA;
    public double dB;
    public uint dwB;
    public uint dwC;
};

[DllImport(@"CPlus.dll"]
private static extern int GetStructArray(out MyStruct structs);

public void Test()
{
    MyStruct[] structs;
    GetStructArray(out structs);
}

I'm even open to returning an array of pointers and using
Marshal.PtrToStructure() if need be.

Thanks so much!

Jason Newell