Question

I need to create a binary blob of empty data to PInvoke a native C++ dll that needs a unsigned char* of nulls.

The native C++ program is expecting a structure of data, and there’s a nulled area of bytes in the middle, but in C# I can’t just make a struct with an initialized byte[] in the middle.

My struct in C++ looks like this

struct myStruct
{
    byte command;
    byte returncode;
    void* Source (a pointer to a string, rather a null term char*)
    void* Destination (same thing)
    byte filler [99]
    byte options;
}

I’ve already figured that I can take a string and convert it to an array of bytes using myStruct.Source = (void*)Marshal.StringtToGlobalAnsi(source) (correct me if I’m wrong).
But I don’t know how to fill out that empty array of bytes in the middle.

This is my C# struct so far.

[StructLayout(LayoutKind.Sequential, Pack = 1)]
unsafe struct Trans_s
{
    public byte command;
    public byte returnCode;

    public void* pSource;
    public void* pDest;
    public byte* filler;
    public byte options;
}

Answer 1

For the array of bytes, you need to mark the field in the C# struct as a byvalarray. The default marshaller uses LPArray if you do not.

[MarshalAs(UnmanagedType.ByValArray, SizeConst = 99)]
public byte[] filler;

You need to allocate the memory for filler when you create an instance of the struct.