How to use COM Interop in C#

Question

I have a requirement to use a dll written in Visual C++ 6.0 in my C# application. In order to establish the link between my application and the dll, I have written a ATL COM Component in Visual C++.NET [visual studio .NET version 8]. This COM component is referenced in my C# application. The COM component statically links with the 6.0 dll. I am facing problems in passing data from C# to the dll through the COM component. Description: 1. The dll exposes a function as: __declspec(dllexport) int __stdcall ProblemFunction(int param1, BYTE param2, BYTE param3, BYTE *param4, DWORD *param5); 2. In the COM wrapper component, I have declared an interface method as: [id(25), helpstring("method ProblemFunc")] HRESULT ProblemFunc([in] SHORT param1, [in] BYTE param2, [in] BYTE param3, [in, out] BYTE* param4, [in, out] LONG* param5, [out, retval] BYTE* param6); where, param6 is used to return the value returned by dll function to the C# client application. The implementation of this interface function is given below: STDMETHODIMP Wrapper::ProblemFunc(SHORT param1, BYTE param2, BYTE param3, BYTE* param4, LONG* param5, BYTE* param6) { *param6 = ProblemFunction(param1, param2, param3, param4, (DWORD*)param5); return S_OK; } 3. In the C# client application, I am calling the COM function as: byte returnValue = DllWrapperObject.ProblemFunc(value1, (byte)value2, (byte)value3, ref value4, ref value5); Here, I am facing problems passing value4 and value5 because: a. For value4 I need to pass a structure. b. For value5 I need to pass the size of the structure being passed in value4. The structure to be passed is as required by the dll. The C++ structure is: typedef struct { DWORD var1; DWORD var2; struct { DWORD var3; char var4[16]; DWORD var5; } var6[3]; } TEST_STRUCT; I wrote the structure's equivalent in C# as a class as I had to create an array. The C# implementation of the above given structure is: internal class InnerStruct { long var3; char[] var4 = new char[16]; long var5; internal static int GetSizeOfClass() { return (sizeof(long) + sizeof(char) * 16 + sizeof(long)); } } internal class TestStruct { long var1; long var2; InnerStruct[] var6 = new InnerStruct[3]; internal static int GetSizeOfClass() { return (sizeof(long) * 2 + InnerStruct.GetSizeOfClass() * 3); } } Questions: 1. Is there any better way to define the structure equivalent in C#? 2. How can I get the size of the structure/class for passing it to the dll? 3. How to pass the handle of the structure/class object from C# to the dll so that it can fill it up and return it to the client app for future usage?