Sunday, September 4, 2011


I started a new job recently that has taken me from a primarily interpreted, dynamically typed existence to one that is primarily compiled and statically typed. More specifically, I have gone from Ruby back to C#. This transition is only around my day job; I still love dynamic languages. But since I spend a good bit of time at work, this has affected a significant chunk of my programming life.

So what is my primary pain point coming back to the .NET world after a couple of years of peace, love, and Volkswagen minibuses? No surprises here; it's Visual Studio. In my day-to-day work I do everything I possibly can in VIM. So light, so immediate. Please don't make me open Visual Studio. It is so slow in everything that it does, even just opening and closing. You know, I'm not always creating a multi-tiered enterprise solution with three UI alternatives and seven WCF services. I very often want to write just one or two lines of code to clarify my understanding of some language behavior or some detail of the framework. It is at these times that I most resent waiting two minutes for the IDE to open. And please don't make me create a new solution just to run one line of code. Can't I just get straight to the language for a second?

One thing that Ruby got me addicted to was the instant feedback afforded by irb. Irb stands for "interactive Ruby." It is nothing unique; it's just Ruby's REPL. If you're not familiar with this term, a REPL is a read-evaluate-print loop. It's a command line shell in which you can type a line of code and have it execute immediately and print the result. For you Visual Studio folks, it's kind of like the Immediate Window, but it doesn't require a paused executing application.

Of course the REPL was not born with Ruby. I think it originated with LISP (before I was born), but you get one with Python, Scala, Haskell, Perl, Prolog—lots of languages. Modern web browsers with good debugging tools provide a REPL for JavaScript, but I prefer the one you get with Node. You might notice that these are all non-Microsoft languages, but my first experience with a REPL was in BASIC, and it was there in the version that came with MS-DOS back in the early 80s.

What is so special about a REPL? If programming is the way we communicate to a computer what we want it to do, must there be so much ceremony around saying anything at all? Writing an application is like writing a book. A small program might be more like a personal letter. But what if you just want to say something, or ask a single question? What's the largest integer value? Can you implicitly cast a float to a Boolean? What happens if I try to add a decimal and a null? What date is ninety days from today? Can you square a list of integers in one line of code? This is where the REPL shines. It's more like a conversation.

If you're like me, you've been spoiled by a REPL in another environment, and now you're spending your days in .NET, you've got to be mourning the loss of that instant feedback. If this a luxury you have never had, I'm telling you now what you've been missing. The good news is that there is a C# REPL in Mono. I don't know why I have not heard more talk about it, but it is a big deal to me. It is totally worth installing Mono for, even if you use it for nothing else.

Just head over to and install Mono. You don't even need MonoDevelop (although it is worth checking out). Add Mono's bin folder to your path, and you've suddenly got this at your disposal:

C# in a REPL. No Visual Studio. No project. And if that's not cool enough for you, it's Mono, so it works on OS X and Linux, too. Use wisely. Enjoy.


  1. Have a look at Linqpad. For me, it does much more.
    You can evaluate single C# statemens, or create a small method. You can evalueate linq Statements, as well as get results back from a Database, in a graphical grid.

  2. Another nice trick is to use Powershell. You can evaluate .NET code right at the command line. Of course, you have to lean the powershell language syntax, so it's not as straight forward as using mono's REPL. But it doesn't require mono and comes with windows.