title: Cannot add source code files to a Windows Forms application?
body: <p>I'm trying add some code from another project into the windows form application project by copying and pasting a folder. (the folder contains some .cs files). However, I cannot see the project in the solution explorer and I cannot use any of those codes. This copy-paste process works for C# console application but it doesn't work for windows form applications.</p>
  <p>How can I add the codes into the project?</p>