Bilal
Bilal

Reputation: 31

How to Call a C# Method from C++ with Parameters?

using System;
using System.Runtime.InteropServices;
using System.Reflection;
public struct StructCreatedByUser
{
    public int x;
    public float anything;
    public string name;
}

class Program
{
    [DllImport("CppLibrary")]
    private static extern void SetPointer(IntPtr ptr);

    [DllImport("CppLibrary")]
    private static extern void CallCppFunctionWithParam(IntPtr param); 


    public static void FunctionCreatedByUser(StructCreatedByUser data){
        Console.WriteLine(data.x + "  " + data.anything + "   " + data.name );
    }

    static void Main()
    {
        StructCreatedByUser data = new StructCreatedByUser { x = 10, anything = 20.5f, name = "wedf" };
        IntPtr param = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(StructCreatedByUser)));
        Marshal.StructureToPtr(data, param, false);
        IntPtr methodPtr = typeof(Program).GetMethod("FunctionCreatedByUser").MethodHandle.GetFunctionPointer();
        SetPointer(methodPtr);
        CallCppFunctionWithParam(param);

    }
}
#include <iostream>
#include <functional>
#include <string>

typedef void (*FunctionStorage)(void*); 
FunctionStorage func;

extern "C" void SetPointer(void* methodPtr) {
    func = reinterpret_cast<FunctionStorage>(methodPtr);
}

extern "C" void CallCppFunctionWithParam(void* param) {
    func(param);  
}

I’m trying to avoid the performance overhead of using MethodInfo.Invoke() in C# Reflection by calling methods directly from C++ using memory manipulation. My goal is to completely bypass Reflection at runtime.

I successfully managed to call a method obtained via Reflection in C++ without passing parameters, but when I tried to pass parameters, I couldn’t make it work. I suspect the user-defined method needs to accept an IntPtr and handle data conversion internally, but this approach is not user-friendly. Ideally, everything related to C++ and memory management should be handled in the background.

Here are my specific questions:

  1. How can I pass parameters (e.g., structs) from C++ to a C# method obtained via Reflection?

  2. When I attempted to pass parameters, I didn’t encounter a compile-time error, but at runtime, the call failed. What could be the likely cause of this issue?

Any guidance or examples on how to implement this would be greatly appreciated.

My .NET version is 8.0.110

Upvotes: 2

Views: 146

Answers (2)

Bilal
Bilal

Reputation: 31

After discussing with CharlieFace in the comments, I learned that what I wanted to achieve can be done without using C++. I want to thank everyone who contributed.

If you found this thread and came here, I recommend reading the comments. They will likely solve your C++ problem as well.

The code that solved my issue is as follows:

using System.Linq.Expressions;
using System.Reflection;

public struct MyStruct
{
    public string name;
}

public class Program
{
    public void FunctionCreatedByUser(int x, float anything, string name, MyStruct myStruct)
    {
        Console.WriteLine($"{x}  {anything}  {name}" + "   " + myStruct.name);
    }

    static void Main()
    {
        Program program = new Program();

        var parameters = new object[] { 10, 20.5f, "TestName", new MyStruct() { name = "asdagdjakfadgkjdhflajg" } };

        MethodInfo methodInfo = typeof(Program).GetMethod("FunctionCreatedByUser");

        var lambdaParam = Expression.Parameter(typeof(object[]), "parameters");

        var methodParams = methodInfo.GetParameters();
        var convertedParams = new Expression[methodParams.Length];
        for (int i = 0; i < methodParams.Length; i++)
        {
            var paramAccess = Expression.ArrayIndex(lambdaParam, Expression.Constant(i));
            convertedParams[i] = Expression.Convert(paramAccess, methodParams[i].ParameterType);
        }

        var methodCall = Expression.Call(
            Expression.Constant(program),  // Instance of the class
            methodInfo,                    // Method to call
            convertedParams                // Parameters for the method
        );

        var lambda = Expression.Lambda<Action<object[]>>(methodCall, lambdaParam);

        Action<object[]> compiledDelegate = lambda.Compile();
        compiledDelegate(parameters);
    }
}

Upvotes: 1

Charlieface
Charlieface

Reputation: 72128

So many issues:

  • Your C++ code expects the function to have a single void* parameter, which means you'd need to pass the struct by reference, as it's too wide for a native int.
  • You also have an obvious memory leak because you are not freeing your HGlobal.
  • GetFunctionPointer() is dangerous, and not intended to be used directly in most cases, as the runtime often needs to set up a trampoline shim depending on the design on the function, the calling convention, and where it's called from. You'd need to use a delegate with Marshal.GetFunctionPointerForDelegate instead.

Honestly I'm very unclear why you're not just letting the marshaller sort this all out for you. Just declare a delegate type with the actual ref StructCreatedByUser and pass that through.

Don't forget to hang on to the delegate using GC.KeepAlive or storing it in a field until you're done with the callback. Also make sure to specify the calling convention on the delegate type.

[DllImport("CppLibrary")]
private static extern void SetPointer(FunctionStorage func);

[DllImport("CppLibrary")]
private static extern void CallCppFunctionWithParam(ref StructCreatedByUser p1);

[UnmanagedFunctionPointer(CallingConvention.Cdecl)]
private delegate void FunctionStorage(ref StructCreatedByUser p1);

private static FunctionStorage _func = FunctionCreatedByUser;  // need to keep this alive

public static void FunctionCreatedByUser(ref StructCreatedByUser data)
{
    Console.WriteLine(data.x + "  " + data.anything + "   " + data.name );
}

static void Main()
{
    StructCreatedByUser data = new StructCreatedByUser { x = 10, anything = 20.5f, name = "wedf" };
    SetPointer(_func);
    CallCppFunctionWithParam(ref data);
}

If you particularly need to generate the delegate via reflection then you can simply do

_func = typeof(Program)
    .GetMethod(nameof(Program.FunctionCreatedByUser))!
    .CreateDelegate<FunctionStorage>();

Upvotes: 2

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