How do you extract classes' source code from a dll file?

Only managed Languages like c# and Java can be decompiled completely.You can view complete source code. For Win32 dll you cannot get source code.

For CSharp dll Use DotPeek becoz it free and works same as ReDgate .Net Compiler

Have fun.

You cannot get the exact code, but you can get a decompiled version of it.

The most popular (and best) tool is Reflector, but there are also other .Net decompilers (such as Dis#). You can also decompile the IL using ILDASM, which comes bundled with the .Net Framework SDK Tools.