dnSpy
dnSpy is a debugger and .NET assembly editor. You can use it to edit and debug assemblies even if you don't have any source code available.
Want to say thanks? Click the star at the top of the page. Or fork dnSpy and send a PR!
The following pictures show dnSpy in action. It shows dnSpy editing and debugging a .NET EXE file, not source code.
Features (see below for more detail)
Debug .NET Framework, .NET Core and Unity game assemblies, no source code required
Edit assemblies in C# or Visual Basic or IL, and edit all metadata
Light and dark themes
Extensible, write your own extension
High DPI support (per-monitor DPI aware)
And much more, see below