C# P\Invoke DLL no entry point into C++?

Question

I have a C++ Dll "TheFoo.dll" with a method "Foo()"

I have access to other C++ code that uses this method by simply calling:

Foo();

I believe the Method does have the:

 __declspec( dllexport )

So, with the reading I've done about P/Invoke, i thought i should be able to simply call the same method from my C# code:

namespace PInvokeExample1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

        }


        [DllImport(@"C:\MyFolder\TheFoo.dll")]
        public static extern
            void Foo();

        private void button1_Click(object sender, RoutedEventArgs e)
        {
            Foo();

        }



    }
}

When i run it, i get an error:

Unable to find an entry point named 'Foo' in DLL 'C:\MyFolder\TheFoo.dll'.

Any ideas why it is not found?

Answer 1

The C++ language supports overloading, much like C# does. You could export an function void Foo(int) and a function void Foo(double). Clearly those two functions could not both be exported as "Foo", the client of the DLL wouldn't know which one to pick. So it is not.

The C++ compiler solves that problem by decorating the name of the function. Adding extra characters that makes a Foo(int) different from a Foo(double). You can see those decorated names by running Dumpbin.exe /exports foo.dll from the Visual Studio Command Prompt, that lists the name of the exported functions. Assuming your declaration was relevant, you'd see ?Foo@@YAXXZ.

So the corresponding declaration in your C# program should be:

    [DllImport("foo.dll", EntryPoint = "?Foo@@YAXXZ", 
               ExactSpelling = true, CallingConvention = CallingConvention.Cdecl)]
    private static extern void Foo();

There are ways to change the C++ function declaration to make the C# declaration easier. Which is actually not a good idea, these decorated names actually help catch mistakes.

Answer 2

If you didn't declare it extern "C" in your dll, its name has likely been "mangled". You can use something like Dependency Walker to see what symbols your dll exports.

Answer 3

You should provide more information on your C++. Try using extern "C" __declspec(dllexport) instead. C++ exports with strange names so using extern "C" avoids that.