Accessing unmanaged memory by address allocated in COM / C++ from C# via Interop

Question

We have a legacy COM DLL with the following simplified IDL for some method:

AllocateMemory( [in] LONG lSize, [out] LONG *pAddr );

Implementation of this method contains the following:

BYTE *pArr = (BYTE*)CoTaskMemAlloc( lSize );  
*pAddr = &pArr;  

The address of newly allocated memory is returned as a long value back to the caller.

C++ clients utilizing this COM object can invoke that method and then access the memory as follows:

//---- C++ ----------------   
long lSize = 10;  
long lAddr;  
pCOMObj->AllocateMemory( lSize, &lAddr );  
byte **bArray = (byte**)lAddr;  
for (int i = 0; i < iSize; i++)  
{  
    printf( "array[%d] = %d\n", i, (*bArray)[i] );  
}  
// Now deallocate memory.  
CoTaskMemFree( *bArray ); 
//--------------------

All works well from C++ COM client.

The same COM DLL was added as a reference to a Visual Studio 2010 project.
Our goal is to be able to do the same from C# client using Interop:

//---- C# ----------------  
int iSize = 10;  
int iAddr = 0;  
objCOM.AllocateMemory( iSize, ref iAddr );  
...    

// ??? *UNKNOWN*  
// At this point we need to be able to access allocated memory pointed to by iAddr.  


// The following does NOT work, i.e., no errors occur but array contents are totally
// different from what has been initialized inside AllocateMemory() method, implying 
// that totally different memory is being accessed/copied:    
byte [] bArray = new byte [iSize];  
IntPtr rAddr = new IntPtr( iAddr );  

// Marshal the array from an unmanaged to a managed heap.  
Marshal.Copy( rAddr, bArray, 0, iSize );  
for (int i=0; i < iSize; i++)  
   Console.WriteLine( bArray[i] );  

// Release the unmanaged array.  
Marshal.FreeCoTaskMem( rAddr );  
//--------------------

Does anyone know of a potential solution?

Answer 1

There is a ReadIntPtr method to read a native width pointer from an address. It will work on both architectures, unlike ReadInt32

Answer 2

Firstly, you must build as only x86- that code will fail on 64bit. Secondly, you must use unsafe code to perform such a conversion in C#.

Answer 3

IntPtr bAddr = new IntPtr( iAddr );  // bAddr = (byte**)iAddr
IntPtr rAddr = new IntPtr(Marshal.ReadInt32(bAddr)); //rAddr = (*bAddr);

byte [] bArray = new byte [iSize];   
// Marshal the array from an unmanaged to a managed heap.   
Marshal.Copy( rAddr, bArray, 0, iSize );   
for (int i=0; i < iSize; i++)   
   Console.WriteLine( bArray[i] );   

// Release the unmanaged array.   
Marshal.FreeCoTaskMem( rAddr );