To find your existing .cs files, right click on the project in your instance of Visual Studio, and click Open Folder in File Explorer.

1. Visual Studio –> Your current project (Windows Form) –> Solution Explorer –> Project Name –> Right Click –> Add –> Existing Item –> Then locate your existing .cs file.

2. Now there’s one last thing to do in order to use the control. Add a using statement to your code, so that your assembly knows about its dependencies.

   using System;
      using System.Collections.Generic;
      using System.ComponentModel;
      using System.Data;
      using System.Drawing;
      .
      .
      .
      using CustomMsgBox; //Here's the using statement for our dependency.
   

3. To display the messagebox, simply use the following…