The file could not be added to the project. This file is within the project directory tree error

Question

Whenever I try to add a file to my Visual Studio project, I get the following warning:

The file [filename] could not be added to the project. This fil

Answer 1

Go in to your Windows File Explorer and navigate to the .csproj file and open it with Notepad (or advanced text editor like NotePad++)

You will notice two lines that have similar or duplicate Compile Include lines:

  "SubDir\MyClass.cs"

Remove the Above line and keep only the line similar to the one below: