System.Access Violation Exception While calling C++ Dll in .Net Application

Question

I am implementing C++ Dll in the C#.

My Wrapper.h file:

`

    class __declspec(dllexport) TestClass
     {   
      public:
              int value;
              TestClass(int value):value(value)
              {
              }
             ~TestClass()
              {
              }
     }

`

My Wrapper.cpp File

   #include "stdafx.h"

   #include "WrapperApplication.h"

My C# code

 public unsafe class Message:IDisposable
{
   private TestStruct* _testStruct;
   private IntPtr* _oldVTable;
      [DllImport(@"WrapperApplication.dll", EntryPoint = "??0TestClass@WrapperApplication@@QAE@H@Z", CallingConvention = CallingConvention.ThisCall)]
   extern static IntPtr Test(TestStruct* testStruct, int value);

   public Message(int value)
   {
       _testStruct=(TestStruct*)Memory.Alloc(sizeof(TestStruct));

       Test(_testStruct, value);
   }
   #region IDisposable Members

    public void Dispose()
    {

        //throw new NotImplementedException();
    }

    #endregion
}

My TestStruct.cs file:

 [StructLayout(LayoutKind.Sequential, Pack = 4)]
  public unsafe struct TestStruct
  {
    public IntPtr* vtable;
    public int value;
  }

I have to Call CPP dll with the help of the Vtable in the .Net Application. I have created TestStruct.cs file as replica of My Cpp class. And trying to Call the CPP constructor in the C# constructor. But at the line Test(_testStruct, value); throws the System.AccessViolation Exception as Attempted to read or Write the memory.It is often indication that other memory is corrupt. The values for _teststruct, value in the Test ctor comes but still it throws the Exception.I have tried many ways but failed to get the solution. Please let me know where I am wrong in the implementaion. So Any Help would Appreciated.

Answer 1

I think the easiest way would be not to call directly a C++-interface DLL from C#. With this premise, two ways appear before you:

• Give your DLL a flat C interface, or a Component Object Model (COM) interface. This will make it callable from most platforms and languages.
• Keep the DLL as-is, but call it from C++/CLI code instead of C# code. It's the reason C++/CLI exists, after all: To make this kind of glue code between .Net Framework applications and unmanaged libraries.

Answer 2

Since you are calling unmanaged code first make sure you Dispose() the unmanaged resources. There is a way you can catch the exception thrown by unmanaged code (if the failing module is your unmanaged component). Decorate your Message() method with HandleProcessCorruptedStateExceptions attribute which will catch any exception thrown by unmanaged code.

  [HandleProcessCorruptedStateExceptions]
   public Message(int value)
   {

    try
      {
         _testStruct=(TestStruct*)Memory.Alloc(sizeof(TestStruct));

         Test(_testStruct, value);
      }
    Catch(AccessViolationException ex)
      {
         //Read the exception here
      }
   }