To compile this, you execute the mono C# compiler:

mcs hello.cs

Which should give us a file named hello.exe -- it's interesting to note that because this file is in IL, it can be run on Windows as well.

Now you can just run the exe:

csharp-helloworld# ./hello.exe 
Hello world

Which implicitly executes mono because my Linux environment has setup a new binary executable type within the kernel. If you Linux box has this turned off, then you can just do what is happening under the hood:

csharp-helloworld# mono hello.exe 
Hello world

Aint that cool?