How To Host Mono’s CSharp Compiler as a Service in .NET - For Runtime Code Evaluation/REPL

By Anoop Madhusudanan

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I Love C#Anders in his C# Futures Talk mentioned about C# Compiler as a Service and demonstrated an REPL (Read Evaluation Print Loop) implementation.

If this is going to be a part of C#/.NET framework, this will allow you to execute plain C# code strings during runtime. This will allow you to implement features like interactive scripting in your applications.

As of now, .NET/C# stack don’t have this. Enter Mono’s C# Compiler as a service implementation. Big Bang. Last April, Miguel announced the availability of Mono’s C# Compiler as a Service assembly, along with a neat REPL C# shell. From that point onwards, I was thinking about writing a quick post on some of my thoughts about hosting Mono’s assembly in your .NET applications, so here we go.

1 – Download the C# Shell and Mono.CSharp.dll assembly

A snapshot of the C# REPL shell and Mono.CSharp.dll is available from this direct link (From Miguel’s post). The Zip contains a C# REPL shell - csharp.exe – and also the Mono.CSharp.dll which is the Compiler as Service assembly that we may use to host the C# compiler in our .NET apps.

2 – Exploring Miguel’s C# REPL Shell

For now, fire up csharp.exe. Once you fire up the shell, type help; to get a list of Static methods available for meta purposes like showing the defined local variables (ShowVars), loading an assembly so that you can use the types inside the same (LoadAssembly) etc.


Also, In the above screenshot of the console, you may see that I’m just assigning some value to variable ‘x’, and then printing it to the console.  Also, see this post from Miguel to learn what you can do more with the C# interactive shell - I know you’ve already fallen in love. Mono’s C# console supports {..} block syntax and some of kind of auto completion as well.

3 – Hosting Mono’s Compiler as a Service in your own .NET applications

Now, let us see how to host the Mono.CSharp.dll in your own .NET applications. Create a console application in Visual Studio, and Add a reference to Mono.CSharp.dll. Now, let us try some simple code.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using Mono.CSharp;

namespace MonoCompilerHost
    class Program
        static void Main(string[] args)
            Console.WriteLine(">Mini Console\n\n");
            while (true) 
                string input = Console.ReadLine();
                if (input == "@@") return;

In the code, you may see that we are importing the Mono.CSharp namespace, so that we can use the Evaluator class in the same. The Evaluator class has a 'Run' static method, that'll take some C# code as string, and execute the same. So, we just wrote a simple REPL leveraing Mono.CSharp.dll, which reads line by line C# code from the console and execute the same. Let us see our Mini Console app in action.


Note that our implementation is not very functional, it is just a ‘hello world’ application to demonstrate how to leverage Mono’s compiler as a service in your own .NET apps.

4 – Advanced Scenarios

There are quite a handful of useful scenarios where you can leverage runtime code evaluation/execution. Like, providing scripting features, allowing your users to write queries using LINQ or so, storing some logic in your application as non compiled, decoupled and plain C# code, that’ll be picked, compiled and executed by your system at run time etc etc. Will blog about that later.

Update: Read Second part of this article Dynamic Filtering and Querying in Your .NET applications - Using Mono’s C# Compiler As Service

You may also read some of my recent posts on 7 Free E-Books For .NET Programmers and Architects and Revisiting Few C# Concepts – Delegates, Anon methods, Expression Trees, Lambdas

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