.NET Reflector / Decompiler

A decompiler is a computer program that takes an executable file as input, and attempts to create a high level source file which can be recompiled successfully.

• https://www.jetbrains.com/decompiler/
• https://github.com/icsharpcode/ILSpy
• https://www.telerik.com/products/decompiler.aspx
• http://www.devextras.com/decompiler/