Question

I’ve created a DLL function for using inside C# using DLLImport but having troubles in calling the method, as I’m getting memory corruption problems;

[DllImport("mydll.dll", EntryPoint = "callinmydll")]
public static extern int testdllcall(double *firstinput, long firstcount, double *secondoutput, long secondcount);

Here’s part of the C++ library header;

extern "C" {              

mydll_API int callinmydll(double *in, long firstcount, double *out, long secondcount);

}

Implementation.

mydll_API int callinmydll(double *in, long firstcount, double *out, long secondcount)
{
    for( int i =0 ; i < 10 ; i++ ) 
    {
        *(out + i) = (*(in + i) + 10 );
    }

    return 0;
}

Now when my DLLImport function calls the callinmydll function and passes valid data to it, this is where things get interesting. The pointer in contains data, as does firstcount. Although everything beyond this point is corrupted. Why? Curiously I rearrange my function to be double, double*, long, long now the corruption happens after third parameter. I’m curious as to what’s going on, as I’m passing valid data; two valid pointers, and int cast to int64.

Help!

Answer 1

In Win32 C, a long is still 32-bits. Your C# signature is using a long which is 64-bits. Your second and forth parameter should be an int in the C# signature.

See this table for more information.

So your signature looks like so:

[DllImport("mydll.dll", EntryPoint = "callinmydll")]
public static extern int testdllcall(double *firstinput, int firstcount, double *secondoutput, int secondcount);

Additionally, make sure your calling convention is correct as Ramhound pointed out in the comments. Your C function looks like it is using the CDecl convention, and .NET defaults to StdCall. You can set the calling convention in the attribute:

[DllImport("mydll.dll", EntryPoint = "callinmydll", CallingConvention = CallingConvention.Cdecl)]