I download open source from sourceforge.net, But I doesn't see .sln. So I can't see source code.

look for a project file. The pattern for the extension will be ##proj. Keep in mind that unless it's presented as a .net project, it's probably written in a different language even if it says that it will interface with .net

The download button only downloads the .dll and an app that uses it, and some documentation. follow the Code tab to download the source. Looks like you might need git for windows you can get one option here

That zip file does not contain the source code. Just the *.exe and some html doc files.

@Ancient Dragon
Hmmm, I think You never download app from sourceforge, it is right ?
Thank you. You make me able to see source code. Very thanks !!!!!!!