Reflector is a class browser for .NET components. It supports assembly and namespace views, type and member search, C# XML documentation viewer, call and callee graphs, IL disassembler, VB and C# decompiler, dependency trees, base type and derived type hierarchies and resource viewers. C#, Visual Basic and Delphi language syntax is supported.

As Chris Sells says it If you're a .NET developer, you must use Reflector and if you use Reflector, you must use Reflector 4.0.!

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