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

mcs dirsize.cs

Which should give us a file named dirtrav.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-dirsize# ./dirsize.exe ..
Total size: 1526282640 bytes
File count: 25557
Directory count: 1658