how to convert class(.cs) file to DLL using ASP.NET

Two ways,
1) Select New project Class Library,
Add a new .cs File and copy your code,
After Building this you get dll in Bin Directory.

2) Us Command Line,

Start Menu> Programs > Visual Studio > visual Studio Tools> VS command Prompt

navaigate where .cs, for example if it is in C:>, then type C:

and type csc /t:library /out:DllName.dll ClassName.cs

Note if you added any referenced assembly add it with /r: