This example shows how to create a simple function that adds two numbers together by creating a simple DLL in C# with Visual Studio 2019 that can be used in Xojo 2020 r1. The two major aspects are to: 1) Install the Microsoft Build Tools 2015, and 2) RGiesecke NuGet package. 

*Uses Xojo API2


Here are the steps to create the plugin in Visual Studio 2019 and read the DLL in Xojo:

  • Download Microsoft Build Tools 2015 and install from:
  • Start Visual Studio 2019
  • Create a New Project
  • Select C# Class Library (.NET Framework) and press Next
  • Create a folder where to store project (I am using C:\test\CSharpXojo\)
  • Project name is CSXojo and press Create
  • Tools->NuGet package manager->Manage NuGet Packages for Solution
  • Browse and choose->Unmanaged Exports by Robert Giesecke->Click on CSXojo in right panel->Select Install
  • Select Release, CPU Manager and choose Configuration: Release, Platform: x64, Build (checked)
  • The following code in Class1.cs should look like the following:



using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

using RGiesecke.DllExport;

namespace CSXojo


    public class Class1



        public static int AddTwo(int x, int y)


            return x + y;






  • Select Build->Build solution
  • Create a new Xojo desktop project
  • Save the project in C:\test and name the project XojoCSharp.xojo_binary_project
  • Select Windows x86 64-bit build
  • Add a label and a button, add action event to button and the following code:


Declare Function AddTwo Lib "C:\test\CSharpXojo\CSXojo\CSXojo\bin\x64\Release\CSXojo.dll" (x as Integer, y as Integer) As Integer

LblAnswer.Text = CStr(AddTwo(145,1023))


Run the 64-bit Xojo program and pressing the OK button will show an answer of 1168. Well done!!!!!

The code is available on Github at: CSharpDLLXojo Github Link