問 ChatGPT 得如下：

To refer to a C# DLL in a C++ project, you typically use a technique called Platform Invocation Services (P/Invoke) or COM Interop. The specific approach you choose depends on whether the C# DLL is a .NET DLL or a COM DLL.

Method 1: Referencing a .NET C# DLL in a C++ Project using P/Invoke:

If your C# DLL is a .NET assembly, you can use P/Invoke to access its functionality from a C++ project. Here are the steps:

1. Compile the C# DLL as a .NET Assembly:
   Make sure your C# DLL is compiled as a .NET assembly with functions marked as extern and decorated with DllImport attribute.

   using System;
   using System.Runtime.InteropServices;
   
   public class MyCSharpClass
   {
       [DllImport("MyCSharpDLL.dll")]
       public static extern int MyFunction();
   }
   

2. Use P/Invoke in C++:
   In your C++ code, you can use the extern keyword to declare the function from the C# DLL and use it as if it were a native C++ function.

   #include <windows.h>
   
   extern "C" {
       int MyFunction(); // Declaration of the C# function
   }
   
   int main() {
       int result = MyFunction(); // Call the C# function
       return 0;
   }
   

3. Compile the C++ Project:
   Ensure that you properly link your C++ project with the C# DLL (.lib file or reference to the DLL).

4. Run the Application:
   Make sure that the C# DLL is in the same directory as the C++ executable or is available in the system's PATH.

Method 2: Referencing a COM C# DLL in a C++ Project:

If your C# DLL is a COM DLL, you can use COM Interop to access it from a C++ project. Here are the steps:

1. Expose Your C# DLL as a COM Component:
   In your C# project, make sure your classes and functions are ComVisible and that you register the assembly for COM Interop. You can do this by adding the [ComVisible(true)] attribute to the relevant classes and registering the assembly using the regasm.exe tool.

2. Generate a Type Library:
   Use the tlbexp.exe tool to generate a type library from your C# DLL. This step is essential for importing the types into your C++ project.

   tlbexp MyCSharpDLL.dll
   

3. Use the Generated Type Library in C++:
   In your C++ project, you can import the type library using the #import directive.

   #import "MyCSharpDLL.tlb" raw_interfaces_only
   
   int main() {
       CoInitialize(NULL);
   
       MyCSharpClassPtr pMyClass(__uuidof(MyCSharpClass));
       pMyClass->MyFunction();
   
       CoUninitialize();
       return 0;
   }
   

4. Compile the C++ Project:
   Ensure that you properly link your C++ project with any required libraries and the COM DLL.

5. Run the Application:
   Make sure the C# DLL (COM DLL) is registered or available on the system.

Choose the appropriate method based on whether your C# DLL is a .NET assembly or a COM DLL.

我之前是用DllExport來讓Java透過JNA呼叫C# dll function，我沒研究過C++