Why is an AccessViolationException thrown when I call an imported C++ function from C# code?

Question

I have been reading many threads about the topic but I have not been able to find a solution to my problem yet.

From my C# code, I try to call a C++ function by using DllImport. Unfortunately, I do not have the C++ code, but the header file gives information about the function:

ABCD Initialize
(
  IN FLOAT a  
  , IN DWORD b  
);  

ABCD is defined in the header file as well:

#define EFGH extern "C"
...
#define ABCD EFGH __declspec(dllexport) HRESULT WINAPI

In C# code, I try to to call the Initialize function like this:

[DllImport("path.dll", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.StdCall)]
public static extern UInt32 Initialize([In] float a, [In] UInt32 b);
...
Initialize(50.0f, 0);

According to this site there is no need for marshalling for the data type float, and instead of DWORD I can use UInt32. For the return type, HRESULT WINAPI, I use the data type UInt32 and in my understanding there no need for marshalling as well? (I also tried to use other data types for parameters and the return value, but always got the same exception)

For the DllImport attribute, many possibilities for CharSet and CallingConventions have been tested, but I had no success. When declaring the function, I also tried both variants, with and without the [In] attribute for the parameters.

When the function is called, an exception is thrown:

System.AccessViolationException: 'Attempted to read or write protected memory. This is often an indication that other memory is corrupt.'

Am I missing something here? What could be my next steps to call the function successfully?

Thank you.

---

Here is the full C# code. As said, I do not have the code that was used to build the .dll file. Also I am not allowed to share the dll.

using System;
using System.Runtime.InteropServices;

namespace MyNamespace
{
  class Program
  {
    [DllImport("path.dll", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.StdCall)]
    public static extern UInt32 Initialize([In] float a, [In] UInt32 b);

    static void Main(string[] args)
    {
      Initialize(50.0f, 0); // <- throws System.AccessViolationException
      Console.ReadLine();
    }
  }
}

Answer 1

Finally, I could determine the problem.

In order to check the data types of the parameters and the return value, I used the decompiler Snowman on the dll. It looked like the dll only defines interfaces, and that the implementations are in another dll file which the developer forgot to send in first place.

Once the second .dll file is stored in the runtime folder of my program, the function call works. I also want to mention that the call works with and without [In] parameter attributes.

While I don't know the exact reason for the thrown exception AccessViolationException: Attempted to read or write protected memory... (I think another exception could fit better here), I am happy that the issue could be solved.

Thanks to Ben Voigt and David Heffernan for their comments!