A few years ago I found myself spending a lot of time writing batch files to perform a variety of relatively simple tasks. For those who aren't familiar, you can do some surprisingly powerful things with batch files - but it usually involves a lot of trickery and arcane knowledge. I wanted a simpler way of doing things, but I didn't want to create a bunch of compiled executables and lose the elegant, easy-to-modify transparency that batch files offer. These days, I'd probably look to PowerShell for the solution, but back then there was no such thing...

What I did have was the power of .NET and some inspiration from a coworker's tool that was able to take .NET 1.1 source code and execute it "on the fly" without the need for compilation. What I did not have was something similar for .NET 2.0 or access to the source code for that tool (because the author had left the company). So I wrote my own:

==  CSI: C# Interpreter                   ==
==  Delay (http://blogs.msdn.com/Delay/)  ==

CSI: C# Interpreter
     Version 2009-01-06 for .NET 3.5

Enables the use of C# as a scripting language by executing source code files
directly. The source code IS the executable, so it is easy to make changes and
there is no need to maintain a separate EXE file.

CSI (CodeFile)+ (-d DEFINE)* (-r Reference)* (-R)? (-q)? (-c)? (-a Arguments)?
   (CodeFile)+      One or more C# source code files to execute (*.cs)
   (-d DEFINE)*     Zero or more symbols to #define
   (-r Reference)*  Zero or more assembly files to reference (*.dll)
   (-R)?            Optional 'references' switch to include common references
   (-q)?            Optional 'quiet' switch to suppress unnecessary output
   (-c)?            Optional 'colorless' switch to suppress output coloring
   (-a Arguments)?  Zero or more optional arguments for the executing program

The list of common references included by the -R switch is:

CSI's return code is 2147483647 if it failed to execute the program or 0 (or
whatever value the executed program returned) if it executed successfully.

   CSI Example.cs
   CSI Example.cs -r System.Xml.dll -a ArgA ArgB -Switch
   CSI ExampleA.cs ExampleB.cs -d DEBUG -d TESTING -R

CSI was inspired by net2bat, an internal .NET 1.1 tool whose author had left
Microsoft. CSI initially added support for .NET 2.0 and has now been extended
to support .NET 3.0 and .NET 3.5. Separate executables are provided to
accommodate environments where the latest version of .NET is not available.

Version History

Version 2009-01-06
Initial public release

Version 2005-12-15
Initial internal release

[Click here to download CSI for .NET 3.5, 3.0, 2.0, and 1.1 - along with the complete source code and test suite.]


Using CSI is quite simple. Here's an example from the release package:

C:\T\CSI>TYPE Samples\HelloWorld.cs
using System;

public class HelloWorld
    public static void Main(string[] args)
        Console.WriteLine("Hello world.");

C:\T\CSI>CSI.exe Samples\HelloWorld.cs
Hello world.

And if you use the included batch files to register the .CSI file type (more on this in the notes below), it's even easier:

C:\T\CSI>TYPE Samples\Greetings.csi
using System;

public class Greetings
    public static void Main(string[] args)
        Console.WriteLine("Hello {0}", string.Join(" ", args));

C:\T\CSI>Samples\Greetings.csi out there world.
Hello out there world.



  • Surprisingly, it's fairly straightforward to do what CSI does thanks to the CSharpCodeProvider class that's part of .NET. (In fact, the majority of the code for CSI is related to processing arguments, managing the display, and handling errors!) Behind the scenes, there's no real magic: CSharpCodeProvider is just a wrapper for the C# compiler which generates a temporary assembly that CSI calls into. Everything should work exactly the same under CSI as it would if compiled normally - the only difference is the convenience. :)
  • I hinted at the benefits of avoiding compilation above, but wanted to explain a bit more. In the usual way of doing things, there is typically (at least) a .CS file and a .EXE file for each tool used by, say a build process. The build process runs the .EXE file, and people refer to the .CS file. If anything ever needs to be changed, someone updates the .CS file, recompiles to generate a new .EXE, and checks both files in together. Usually... But sometimes they forget and only check in the .EXE - or there's some other .EXE-only modification that throws the synchronization of the two files out of whack and then it's just a mess to understand what's going on because the source code no longer matches the executable. CSI avoids that problem by allowing the build process to "run" the .CS file directly. Because there's only one file to manage, there's no chance of it getting out of sync!
  • Two simple scripts are included that can be used to RegisterCSI.cmd or UnregisterCSI.cmd. When run with administrative privileges, these scripts set up (or remove) a file association for .CSI files (which are just renamed .CS files) that automatically invokes CSI for the specified file. So, as shown in the earlier example, it's possible to run C# code files by name in a command window or by double-clicking them in the Windows Explorer!
  • There's a separate version of CSI compiled for each major .NET version that's in use today: 1.1, 2.0, 3.0, and 3.5. Aside from some minor functional limitations with the 1.1 version (due to non-existent features in that version of .NET), they're all the same CSI with the same options, etc.. The most obvious difference is the list of default assemblies included by the -R option: each version matches what Visual Studio 2008 provides as the default set of assemblies for a new project targeting the corresponding version of the Framework. The other big difference is that the .NET 3.5 version of CSI uses the C# 3.0 compiler (which can be a little tricky to do until someone shows you how), so it also supports new language features like var and all the LINQy syntax goodness that's available in .NET 3.5.
  • Editing standalone .CS files can be a bit annoying because the lack of an associated .CSPROJ file means that Visual Studio's IntelliSense isn't available. So I'll usually begin by creating a project in Visual Studio, taking advantage of its rich editing and debugging facilities to get everything working the way I want - then I pull out the resulting .CS file and run it with CSI. If there are any subsequent modifications to the code, they're usually trivial enough that the lack of IntelliSense isn't an issue. Alternatively, you could create a new project and add a link to the existing file in order to enable IntelliSense. OR you could use something like Snippet Compiler which offers a surprisingly rich Visual Studio-like editing and execution environment (with IntelliSense!) and works well with standalone .CS files.
  • Given that I've already mentioned PowerShell, what's the point of CSI? Well, maybe there isn't one... :) But I think there's still value here - even in a PowerShell world. (In fact, what prompted me to release CSI was an internal request to add support for .NET 3.5, so it seems at least one other person agrees with me.) Aside from knowing a bit about PowerShell and how it works, I haven't used it, so I won't try to do a feature comparison here. Some of the things PowerShell does are very cool and quite useful - and I think it's considerably better than .CMD files for automating typical tasks. Also, I love that it's tightly integrated with the .NET Framework. That said, it's another "language" to learn and not everybody has the time for that. Additionally, I don't think the current development and debugging options are quite as rich as Visual Studio already is. And PowerShell isn't available everywhere, whereas CSI offers a no-touch, install-free solution anywhere the .NET Framework can be found. Lastly, CSI is small (really small!) and very simple.
  • Though its name bears passing resemblance to a popular TV series, that's unintentional: once you know the C# compiler is named CSC.exe, it's obvious the C# interpreter should be named CSI.exe.


CSI was a fun project in its day and I've done quite a bit with it that would have been fairly tedious otherwise. Today's world offers plenty of alternatives - but if you're comfortable with C# and prefer to stick to one language for all your programming needs, CSI is pretty handy to have around. Because you never know when the urge to code will strike! :)