Can't seem to get a non-zero IntPtr from a byte[] instance variable

Question

I've found many ways of getting an IntPtr from a byte[], all of which I could successfully pass into external unmanaged code, but only if I allocate the byte[] on the stack. Attempting to do the same on a byte[] instance variable, I get a null (IntPtr.Zero) result, no matter which method of getting an IntPtr I choose. I haven't found any information on whether or not instance variables are treated differently than those allocated on the stack, in this case.

Here is what I would like to use to acquire a valid IntPtr to a byte[] instance variable:

GCHandle pinned = GCHandle.Alloc(outBytes, GCHandleType.Pinned);
IntPtr ptr = pinned.AddrOfPinnedObject();

// Always true, for reasons I'm unaware.
if (ptr == IntPtr.Zero) {}

pinned.Free();

Thanks!

Answer 1

The only time that GCHandle.Alloc( thing, GCHandleType.Pinned ) results in a handle to IntPtr.Zero is when thing is null.

Your byte array reference is null when you provide it to GCHandle.Alloc().

Here's where it returns zero:

public class ZeroTest
{
    private byte[] someArray;

    public Test()
    {
        this.someArray = null;
    }

    public void DoMarshal()
    {
        GCHandle handle = GCHandle.Alloc( this.someArray, GCHandleType.Pinned );

        try
        {
            // Prints '0'.
            Console.Out.WriteLine( handle.AddrOfPinnedObject().ToString() );
        }
        finally
        {
            handle.Free();
        }
    }
}

Here's where it returns non-zero:

public class Test
{
    private byte[] someArray;

    public Test()
    {
        this.someArray = new byte[1];
    }

    public void DoMarshal()
    {
        GCHandle handle = GCHandle.Alloc( this.someArray, GCHandleType.Pinned );

        try
        {
            // Prints a non-zero address, like '650180924952'.
            Console.Out.WriteLine( handle.AddrOfPinnedObject().ToString() );
        }
        finally
        {
            handle.Free();
        }
    }
}