How to open .dll files to see what is written inside?

I lost the solution of class-library, can I open .dll file which is created by class-library.

you are better off with a decompiler like redgates .net reflector or jetbrains resharper decompiler. there are open source ones also like

http://www.jetbrains.com/decompiler/

http://ilspy.net/

Follow below steps..

1. Go to Start Menu.
2. Type Visual Studio Tool.
3. Go to the folder above.
4. Click on "Developer Command Prompt for VS 2013" in the case of VS 2013 or just "Visual Studio Command Prompt " in case of VS 2010.
5. After command prompt loaded to screen type ILDASM.EXE press ENTER.
6. ILDASM window will open.Drag the .dll file to window from your folder.Or click on File->New.Then Add required .dll file.
7. After above steps Mainfest and .dll file will appear. Double click on these files to see what it contains.