The following is the command to compile dllsource.cs as a dynamic link library instead of the executable file:
$ mcs /target:library dllsource.cs
We can also specify a different name of the binary with the /out option. For example:
$ mcs /target:library /out:mylibrary.dll dllsource.cs
Finally, if we want to compile a souce which depends on a dll file, we need to use the /reference option.
$ mcs example.cs /reference:mylibrary.dll
Note: mcs is the mono compiler. If you use Microsoft C# compiler, just replace mcs with csc.