Decompile ASP.NET dlls

Use ILSpy

Open ILSpy, and clear out the Assemblies window (Ctrl-A, delete)
Select the .dlls containing your source code and Drag them into ILSpy’s Assemblies window
Select all of the Assemblies in ILSpy (Ctrl-A)
File -> Save Code. 
ILSpy will generate a Visual Studio .sln file containing all the code for you – too easy!