C++

#include "pch.h"
#include <iostream>
using namespace std;
struct Data
{
	double data[1];
};
extern "C"  __declspec(dllexport) void Add(double a, double b, Data * pData)
{
	double c = a + b;
	pData->data[0] = c;	
}

C#

using System;
using System.Threading;
using System.Runtime.InteropServices;
namespace ConsoleApp
{
    struct Data
    {
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 1)]
        public double[] data;
    };
    class test
    {
        [DllImport("Dlldemo.dll", EntryPoint = "Add", CallingConvention = CallingConvention.Cdecl)]
        public extern static void Add(double a, double b,  ref Data pData);
        private static volatile object obj = new object();
        static public void Addresult(double a, double b , ref Data sd)
        {
            lock (obj)
            {
                sd.data = new double[1];
                Add(a, b, ref sd);
            }
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            Data sd = new Data();
            Thread thread1 = new Thread(delegate() {
                test.Addresult(1.2, 5.5, ref sd);
                double ids = sd.data[0];
                Console.WriteLine(ids);
            });
            Thread thread2 = new Thread(delegate() {
                test.Addresult(2.2, 6.9, ref sd);
                double ids = sd.data[0];
                Console.WriteLine(ids);
            });
            thread1.Start();
            thread2.Start();
            Console.ReadKey();
        }
    }
}