1. system platform
   win10 64bit
2. Software version
   Visual Studio: VS2017
   MATLAB: MATLAB R2018b
   Note: the version of MATLAB is not lower than VS, otherwise the call fails.
3. Set up MATLAB external interface
   (1) select compiler
   >>mbuild -setup
   

(2) generate DLL file
Method 1:
>>deploytool
Select Library Compiler:

Method 2:
Select directly from the above APP, Library Compiler:

①: C call selection:. NET Assembly
②: click the plus sign to select one or more m files (at this time, multiple M files are integrated into one dll);
③: the name of the generated DLL, which is the name of the first m file by default, can be modified to another name;
④: Class Name is the Class1 used in the call, which can also be modified;
⑤: it has been proved that MCR needs to be installed when running on other computers;
⑥: click Package to generate the file.
Generate the following files and select the dll file in the second folder.

(3) prepare C-program:
Add two dependent libraries: MWArray.dll and MATLAB dll.dll.

Add reference → browse → select the two files respectively → OK.

Add namespace:

using MathWorks;
using MathWorks.MATLAB;
using MathWorks.MATLAB.NET;
using MathWorks.MATLAB.NET.Arrays;
using MathWorks.MATLAB.NET.Utility;
using MATLAB_DLL;

Programming: (since the MATLAB version is 64bit, x64 is selected as the compiling platform)
① when outputting a parameter:

function result=myadd(a,b)
result=a+b;
end

② when multiple parameters are output:

function [res_add,res_sub,res_mul,res_div]=cal(a,b)
res_add=a+b;
res_sub=a-b;
res_mul=a*b;
res_div=a./b;
end

The specific procedures are as follows:

using System;
using MathWorks.MATLAB.NET.Arrays;
using Matlab_dll;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            //Load Class1 in MATLAB DLL
            Matlab_dll.Class1 matdll = new Matlab_dll.Class1();
            //When the output parameter is one
            MWArray a = 1, b = 2, c;//When MWArray is calling MATLAB, the parameter type is MWArray.
            c = matdll.myadd(a, b);//Call myadd
            Console.WriteLine("When outputting a parameter:");
            Console.WriteLine("a + b = ");
            Console.WriteLine(c);

            //When the output parameters are multiple
            MWArray[] input = new MWArray[] { a, b };//Put the input parameter in a MWArray [].
            MWArray[] output = new MWArray[4];//Set the number of output parameters
            matdll.cal(4, ref output, input);//The first parameter indicates the number of outputs
            Console.WriteLine("When multiple parameters are output:");
            Console.WriteLine("a + b = ");
            Console.WriteLine(output[0]);//Output first result
            Console.WriteLine("a - b = ");
            Console.WriteLine(output[1]);//The second
            Console.WriteLine("a * b = ");
            Console.WriteLine(output[2]);//Third
            Console.WriteLine("a / b = ");
            Console.WriteLine(output[3]);//Fourth

            //If you need to use the call result for other calculations
            MWNumericArray re_tmp = (MWNumericArray)output[0];
            double[,] re = new double[1, 1];//MATLAB deals with matrix. The defined array to be assigned cannot be one-dimensional
            re = (double[,])re_tmp.ToArray();
            double xx = 10;
            xx += re[0, 0];
            Console.WriteLine("");
            Console.WriteLine("xx:{0:2f}", xx);

            Console.ReadKey();
            
        }
    }
}

The results are as follows:

Note:
1. Loading Class1 of MATLAB DLL is slow;
2. The compilation platform is consistent with MATLAB;
3. Call MATLAB function operation is very fast.