How to call managed code from unmanaged code?

I believe you are looking for Reverse PInvoke. If you google for reverse pinvoke you'll get a lot of helpful entries. I think the following has a good quick and dirty example.

PInvoke-Reverse PInvoke and __stdcall - __cdecl

Assuming you are talking about real unmanaged code - not just native C++ running in a mixed-mode assembly compiled with /clr - the easiest way is to create a wrapper to your .NET code in C++/CLI. You can then export the C++/CLI methods by just marking them with __declspec(dllexport).

Alternatively, if you have control over the invocation of the unmanaged code, you can marshal function-pointers to your .NET methods and pass them to the unmanaged code.

Take a look at the GCHandle class and the gcroot keyword, which provides a typesafe, templated wrapper around GCHandle.

You can use these to hold a reference to a CLR object (or a boxed value) in native code.

MSDN has a basic tutorial here.

Look at this solution: https://sites.google.com/site/robertgiesecke/Home/uploads/unmanagedexports The solution allows to call C# function from C by decorating your function with [DllExport] attribute (opposite of P/Invoke DllImport).

Exmaple:

C# code

class Test
{
     [DllExport("add", CallingConvention = CallingConvention.StdCall)]
     public static int Add(int left, int right)
     {
         return left + right;
     } 
}

C code:

 extern "C" int add(int, int);

 int main()
 {
      int z = add(5,10);
      printf("The solution is found!!! Z is %i",z);
      return 0;
 }

Output:

The solution is found!!! Z is 15

Update: There is a question and a good answer in comments:

How do I include the dll in the unmanaged project?

You have to link to the .lib file that is generated upon compiling your C# code (https://msdn.microsoft.com/en-us/library/ba1z7822.aspx?f=255&MSPPError=-2147217396)