Passing variable length array of bytes from C# to unmanaged C++ dll

Question

I have a COM dll written in unmanaged C++ that I call from C#. There is a method I call that I pass a buffer to that it then fills in. When it is fixed length it works, when it's variable length it fails with a "access outside of array bounds" error.

Here is the fixed length that works:

C#

PATTERNSLib.PatternDraw p2 = new PATTERNSLib.PatternDraw();

Byte[] buf = new Byte[X * Y * 32 / 8];   // X=756, Y=360 in this case

p2.DrawIntoBuffer(buf, X, Y);

IDL

[id(53), helpstring("method DrawIntoBuffer")]
HRESULT DrawIntoBuffer([in, out] unsigned char buf[756*360*32/8], [in] int width, 
                        [in] int height);    // size hard coded which is a problem

C++

STDMETHODIMP CPatternDraw::DrawIntoBuffer(unsigned char *buf, int width, int height)

Here is my attempt at a variable length array that fails:

C#

PATTERNSLib.PatternDraw p2 = new PATTERNSLib.PatternDraw();

Byte[] buf = new Byte[X * Y * 32 / 8];   // Goal is variable length

p2.DrawIntoBuffer(ref buf[X * Y * 32 / 8], X, Y);   // compiler error indicated ref was required

IDL

[id(53), helpstring("method DrawIntoBuffer")] 
HRESULT DrawIntoBuffer([in, size_is(width*height*32/8), out] unsigned char *buf, 
                       [in] int width, [in] int height);

C++

STDMETHODIMP CPatternDraw::DrawIntoBuffer(unsigned char *buf, int width, int height)

Answer 1

Don't do

p2.DrawIntoBuffer(ref buf[X * Y * 32 / 8], X, Y); 

Since this is sending a reference (pointer) to the memory after the array.

Do

p2.DrawIntoBuffer(ref buf[0], X, Y); 

This will send a reference (pointer) to the first element in the array.