PInvoke struct with array of unspecified length

Question

C definitions

typedef struct {
    const uint8_t* buf;
    int bufLen;
} Info;


int Foo(Info* info);

C# definitions

[StructLayout(LayoutKind.Sequential)]
public struct Info
{
     // [MarshalAs( ??? )]
     public byte[] buf;
     int bufLen
}    

[DllImport(...)]
public static extern int Foo(ref Info info);

I'm having trouble figuring out what to specify for the MarshalAs attribute on byte[] buf in the C# struct definition. The buffer is allocated on the .NET side, and its length is known at call time.

In a simple little test:

var info = new Info {
    buf = new byte[] {0x40, 0x50, 0x60, 0x70},
    bufLen = 4,
};

Foo(ref info);

Everything seems to be working correctly, but in fact the data in my buffer is incorrect. Printing it out from the DLL I see 01 00 80 00 - not sure what that is.

I've tried:

• No MarshalAs
• [MarshalAs(UnmanagedType.SafeArray)]

and nothing works.

In general, I really don't know the best way to debug these kinds of problems either.

Answer 1

Per Hans Passant's suggestion, I implemented the following:

[StructLayout(LayoutKind.Sequential)]
public struct Info : IDisposable
{
    private IntPtr buf;
    private int bufLen;

    public Info(byte[] buf) : this() {
        this.buf = Marshal.AllocHGlobal(buf.Length);
        Marshal.Copy(buf, 0, this.buf, buf.Length);
        this.bufLen = buf.Length;
    }

    public void Dispose() {
        if (buf != IntPtr.Zero) {
            Marshal.FreeHGlobal(buf);
            buf= IntPtr.Zero;
        }
    }
}