Unable to Call Simple Custom Dll Import from C# Application

Question

Update

I have now run through the tutorial 3 times with the same results (Visual Studio/Windows is unable to recognise the DLL as a valid file). This must be an issue with my environment, as mike below ran through the tutorial and it worked fine.

I did notice this in the C++ project's solution explorer on my last run:

Does anyone know if this is okay? There seems to be a lot of Red stop-signs which to me would suggest something bad...

---

Following this tutorial, I have ended up with the following:

C#

using System.Runtime.InteropServices;

namespace TestImport
{
    class Program
    {
        [DllImport("TestLib.dll")]
        public static extern void DisplayHelloFromDLL();

        static void Main(string[] args)
        {
            DisplayHelloFromDLL();
        }
    }
}

C++

In .cpp file Source Files

#include <stdio.h>

extern "C"
{
    __declspec(dllexport) void DisplayHelloFromDLL()
    {
        printf("Hello from DLL !\n");
    }
}

When debugging in Visual Studio:

An unhandled exception of type 'System.DllNotFoundException' occurred in TestImport.exe

Additional information: An attempt was made to load a program with an incorrect format. (Exception from HRESULT: 0x8007000B)

When running the built EXE (from he same folder as the Dll):

An attempt was made to load a program with an incorrect format.

Have I done anything wrong in the above code? I did do all of this in VS 2015, where the tutorial calls for VS 2010, howeve rI couldn't find a more up-to-date tutorial.

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When I try to add the project as a reference by Project -> Add Reference ... I am seeing this error:

A reference to 'Path\To\MyLib.dll' could not be added. Please make sure 
that the file is accessible, and that it is a valid assembly or COM
component.

Which makes me think that the issue is either with the source code or the DLLs configuration.

I have also tried building both files and putting them in the same folder before running the Executable, but without any improved results (same error as when running the EXE above).

---

Solution Explorer Screenshot:

Answer 1

Make sure the name of your C++ project and DllImport parameter are the same. Also, if the dll file is not located in the same directory as your C# project, make sure to add the dll into your Debug/Release folder.

Also! Make sure that you are running in the same 32/64 bit mode when compiling.. if you compiled the dll using 32 bit and try to use it in a 64 bit C# program, it won't work.

Answer 2

OK, now build this application, and then copy the previously built DLL into the Debug/Release directory of the current application . The DLL should be in the same directory as your main application.

Did you do the bolded text? Your C# app won't know where to look for the DLL and it looks in the build folder, by default.

Answer 3

An unhandled exception of type 'System.DllNotFoundException'

It happened to me, because of the imported c++ dll, its own dependency dlls were missing.

After copying them into Debug/Release folder everything worked.