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.

Some Features 

• Debug .NET Framework, .NET Core and Unity game assemblies, no source code required
• Set breakpoints and step into any assembly
• Locals, watch, autos windows 
• All metadata can be edited
• Edit methods and classes in C# or Visual Basic with IntelliSense, no source code required
• Add new methods, classes or members in C# or Visual Basic
• IL editor for low level IL method body editing