P/Invoke C# struct with strings to C void*

Question

I'm having a problem creating a C# P/invoke wrapper around a third party C library. In particular, the library has a method with the signature

int command(SomeHandle *handle, int commandNum, void *data, int datasize);

It is a wildcard method that does different things depending on commandNum. data can be a pointer to anything, like a single integer, or a char[], or a struct of some kind (my problem).

I have declared the wrapper as follows:

[DllImport("LIBRARY.DLL", EntryPoint = "command")]
public static extern int Command(IntPtr Handle, int CommandNum, [In, Out] IntPtr Data, int DataSize);

Now, when i call it with an opcode to fill a byte[] it works:

//WORKS, Buffer contains "library 1.0" after the call
const int BUFFER_SIZE = 128;
byte[] Buffer = new byte[BUFFER_SIZE];
int BytesWritten = 0;
GCHandle BufferHandle = GCHandle.Alloc(Buffer, GCHandleType.Pinned);
try
{
    BytesWritten = Command(MyHandle, GET_VERSION, BufferHandle.AddrOfPinnedObject(), BUFFER_SIZE);
}
finally
{
    BufferHandle.Free();
}

However, when I try the same with a simple struct, I cannot make it work, no matter what I try. the struct looks like this:

public struct FormatInfoType
{
    public int Format;
    public IntPtr Name; //const char* 
    public IntPtr Extension; //const char* 
}

Here I am supposed to fill "Format" with an int (say, 1) and then the call to "command(...)" is meant to give me back the name and extension fields

If I pass this struct, the code compiles and runs correctly, but the values in the struct are never modified. If I change the IntPtr's to Strings or StringBuilders (and I've tried a myriad of MarshalAs attributes), then I cannot get the IntPtr to the struct because the it becomes non-blittable and the GCHandle line throws an exception.

Any help on this would be greatly appreciated.

EDIT:

I've tried many ways to call command() with the structure, but currently it looks like this:

FormatInfoType f = new FormatInfoType();
f.Format = 1;
f.Name = IntPtr.Zero;
f.Extension = IntPtr.Zero;

GCHandle fHandle = GCHandle.Alloc(f, GCHandleType.Pinned);
try
{
    Command(MyHandle, GET_FORMAT_INFO, fHandle.AddrOfPinnedObject(), Marshal.SizeOf(f));
}
finally
{
fHandle.Free();
}

Answer 1

You can overload p/invoke signatures, try:

[DllImport("LIBRARY.DLL", EntryPoint = "command")]
public static extern int Command(IntPtr Handle, int CommandNum, ref FormatInfoType Data, int DataSize);