Implementing a C# interface with multi dimensional arrays in C++

[Tventano]

An exisitng C# interface reads:

public interface IGridBand
{
void GetDataBlock(GridRange block, Byte[,] values);
void GetDataBlock(GridRange block, Int16[,] values);
void GetDataBlock(GridRange block, Int32[,] values);
// (etc.)
}

To reuse old code I want to implement this interface in (managed) C++:

public __gc class RasterBand: public IGridBand
{
public:
void GetDataBlock(GridRange block, Byte values __gc[,]);
void GetDataBlock(GridRange block, Int16 values __gc[,]);
void GetDataBlock(GridRange block, Int32 values __gc[,]);
// (etc.)
}

The compiler nor linker complains.

Intended use is from C#. But as soon as a module is loaded which calls a
function which returns a RasterBand I get a exception of type
'System.TypeLoadException' Additional information: Method GetDataBlock in
type xxx.RasterBand from assembly xxx, Version=1.0.1599.31291,
Culture=neutral, PublicKeyToken=null does not have an implementation.

What is happening?