Attempting to access byte array in C# from C++ Dll

Question

So I am trying to push an unsigned char array from a C++ DLL and wanting to access it in a C# GUI. I set up the C# as an IntPtr and I am trying to Marshal.Copy the byte array but the IntPtr is always NULL, what amI diong wrong!

Any help would be greatly appreciated.

Below is the code:

Portion of the C++ DLL Header

class TEST_API TestClass
{
public:
TestClass(void);
~TestClass(void);

void GetData( unsigned char * testData, unsigned size );

unsigned char testInfo[4096];
};
#ifdef __cplusplus
extern "C" {
#endif
extern TEST_API TestClass* CreateTestClass();
extern TEST_API void DisposeTestClass(TestClass* pObject);
extern TEST_API void TestGetData( TestClass* pObject, unsigned char * testData );
#ifdef __cplusplus
}
#endif

This is the C++ DLL portions, it was too much code to put the whole thing

extern "C" TEST_API void TestGetData( TestClass* pObject, unsigned char * image )
{
   if(pObject != NULL)
      pObject->GetData( image );
}
void TestClass::GetData( unsigned char * testData )
{
   for ( int i=0; i < 64; i++ )
      for ( int j=0; j < 64; j++ )
             testInfo[j + i*64] = j;
   testData = testInfo;
}

C# code I did is the follow:

IntPtr trial = new IntPtr();
testing.RetrieveTestData(trial, 64, 64);

byte[] managedArray = new byte[64*64];
Marshal.Copy(trial, managedArray, 0, 64*64);

for (int i = 0; i < 64; i++)
   for (int j = 0; j < 64; j++)
      Console.WriteLine(managedArray[j + i * 64]);

Basically the trial IntPtr is 0 everytime

I hope someone can help me through this. Thanks

Answer 1

trial is passed to RetrieveTestData as an input parameter - meaning it cannot be changed by the caller. To be changed by the caller the parameter must be either ref or out