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.
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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 requiredEdit assemblies in C# or Visual Basic or IL, and edit all metadataLight and dark themesExtensible, write your own extensionHigh DPI support (per-monitor DPI aware)And much more, see belowdnSpy uses the ILSpy decompiler engine and the Roslyn (C# / Visual Basic) compiler and many other open source libraries, see below for more info.
dnSpy-net472.ziprequires .NET Framework 4.7.2 or later.
dnSpy-netcore-win32.zipanddnSpy-netcore-win64.zipdon't depend on .NET Framework, but use .NET Core instead which is included in the zip files. IfdnSpy.exefails to start, try: