c function that return a struct to c# RRS feed

  • Question

  • Hello,

    i have a c function that return a structure, i have build a .dll and i want to call it from c# like this:

    typedef struct predtionresult { float prediction[5]; char *name[5]; }PREDICTIONRESULT; PREDICTIONRESULT predict() { PREDICTIONRESULT predres ; for(i = 0; i < 5; ++i){[i]= names[i]; //names[i] are not empty predres.prediction[i] = predictions[i] // prediction are not empty there is

    //some part of code that i can't share } return predres; } i have declared the c# structure like this: [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi, Pack = 1)] public struct PREDICTIONRESULT{ [MarshalAs(UnmanagedType.ByValArray, SizeConst = 5)] public float[] prediction; [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 5)] public string[] name; };

    and call the function like this:

    PREDICTIONRESULT prediction;
    prediction = predict();

    The problem is the prediction array and the name array are empty, i didn't get the returned value from the c function, so i think that my c# structure have a problem that i didn't get.

    Thank you 


    Friday, May 18, 2018 12:08 AM

All replies

  • Hi Read lafi,

    Thank you for posting here.

    I want to make it according to your C++ code. But I am not good at C++. There are some complier errors in your C++ code. Hence I make a simple example for your reference.

    My C++ dll

    // MyDll.cpp : Defines the exported functions for the DLL application.
    #include "stdafx.h"
    #define EXPORT_DLL extern "C" __declspec(dllexport) 
    EXPORT_DLL int Add(int a, int b)
    	return a + b;
    struct Bar
    	int id;
    EXPORT_DLL void GetBar(Bar& bar)
    	//Bar b; = 10;

    My C# code

    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Runtime.InteropServices;
    using System.Text;
    using System.Threading.Tasks;
    namespace ConsoleApp
        public struct Bar
            /// int
            public int id;
        class CSharp_call_C__
            [DllImport(@"D:\Visual Studio 2017\Projects\ConsoleApp\Debug\MyDll.dll", EntryPoint = "Add", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
            extern static int Add(int a, int b);
            [DllImport(@"D:\Visual Studio 2017\Projects\ConsoleApp\Debug\MyDll.dll", EntryPoint = "GetBar", CharSet = CharSet.Auto, CallingConvention = CallingConvention.Cdecl)]
            extern static void GetBar(ref Bar bar);
            static void Main(string[] args)
                int A = Add(1, 2);
                Console.WriteLine(Add(5, 4));
                Bar bar = new Bar();
                GetBar(ref bar);
                Console.WriteLine("BAR id is " +;

    Hope it would be helpful.

    Best Regards,


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    Sunday, May 20, 2018 2:55 PM