Problem C++ handle 2D array of C#

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C++

C#

Hi All !

I have a 2D array in C# and want handle it by c++.
I write code for C++:

extern "C"
{
     __declspec (dllexport) void __stdcall MatrixH(unsigned char **mymatrix,int matwidth,int matheight)
     {
          mymatrix[0][0]++;
     }
}

and compile it to mylib.dll.

And my code in C# :

static byte[,] mymat = new byte[10,10];   // my 2d array

[DllImport("mylib.dll")]
extern static void MatrixH(byte[,] mymatrix,int matwidth,int matheight);

private static void MyHandle()
{
     MatrixH(mymat,10,10);
}

When I run my code, a error was appear in function "MatrixH(mymat,10,10)":
"Attempted to read or write protected memory.This is often an indication that memory is corrupted."

Please! help me :(