how to convert ddl file into source code

hi,

you may use .NET Reflector to your dll into C# or VB code.

there are so many third party Reflector available
please refer

http://tedgustaf.com/blog/2011/4/ilspy-is-a-free-alternative-to-reflector-for-disassembly/
http://www.red-gate.com/products/dotnet-development/reflector/

hope this will help you.