Question

I have a very simple function in an unmanaged DLL, but I’m not getting the correct return value back from it.

I can confirm that the general PInvoke mechanism is working with one function in my C DLL:

/* Return an integer */
extern "C" __declspec(dllexport) long get_num()
{
    return 42;
}

I call the above unmanaged entry point like so from C# .NET:

[DllImport("My_C_DLL.dll")]
extern static long get_num();
// ...
long ans = get_num();
Console.WriteLine("The answer is {0}.", ans);

This works fine, but passing marshalled parameters to another function in the DLL returns a wrong result:

/* Add two integers */
extern "C" __declspec(dllexport) long add_num(long a, long b)
{
    long sum = a + b;

    return sum;
}

Called from C# as:

[DllImport("My_C_DLL.dll")]
extern static long add_num(long a, long b);

long a = 6, b = 12;
long sum = add_num(a, b);
Console.WriteLine("The answer is {0}.", sum);

This gives me back a result of “6”, or whatever I set the input value of a to be.

I’m guessing that some incorrect marshalling of the input values is messing up the call stack, resulting in the bad return value, but where is the error?

Answer 1

Two problems here. First of all C# long does not match C long. On Windows, C long is 32 bits. Use int in your C# code to match up with your C long.

The other problem is that the calling conventions probably don’t match. You most likely have cdecl in your C DLL but the C# default is stdcall. Fix this by changing your p/invoke.

[DllImport("My_C_DLL.dll", CallingConvention=CallingConvention.Cdecl)]