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C# As a Scripting Language in your .NET Applications – Using Mono’s Compiler As a Service

image Note: This is the third article in the series. In the first two articles, I explained

Preface

In this post, we’ll see how to use C# as a scripting language in your .NET applications. Let us start with a very minimal game application, where you have two players, and your users can control these players by writing some script.

In the previous post, I explained how to create a delegate/predicate from Mono’s Evaluator and pass it back to the Main application, so that we can use the predicate for custom filtering operations. Now, What is interesting in below code is, how we are passing the context information from the main application to Mono’s Evaluator, so that the objects from the main applications can be accessed via Mono’s Evaluator for scripting.

C# For Scripting

So here we go. Have a look at this entire implementation first. Fire up a console project in Visual Studio, add a reference to Mono.CSharp.dll (See the first post), and try this code. (You can click ‘Copy to clipboard’ for copying the code from the listing below).

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

namespace MonoHost
{
    // A simple extension class with methods to 
    // invoke Evaluator methods on a string
    public static class EvaluatorExtensions
    {
        public static T Eval<T>(this string code) where T : class
        {
            return Evaluator.Evaluate(code) as T;
        }

        public static void Run(this string code, bool repQuotes = false)
        {
            var run = repQuotes ?
                        code.Replace("'", "\"") : code;
            Evaluator.Run(run);
        }
    }

    //A simple player class so that we can create few 
    //player objects later
    public class Player
    {
        public string Name { get; set; }

        public void Attack(Player target)
        {
            Console.WriteLine(Name + " is attacking " + target.Name);
        }

        public void Jump()
        {
           Console.WriteLine(Name + " is jumping");
        }

        public void Goto(int x,int y) 
        { 
           Console.WriteLine(Name + " is going to " + x + "," + y);
        }
    }


    // A Context class to share context between our ScriptDriver and Mono's Evaluator 
    public static class Context
    {
        static Context()
        {
            Items = new Dictionary<string, object>();
        }
        public static Dictionary<string, object> Items { get; set; }
    }


    //Main Program
    class ScriptDriver
    {
        static void Main(string[] args)
        {
            //Initializing the evaluator
            Evaluator.Init(new string[0]);

            //Step 1 - Add this assembly as a reference so that we can access the Context class
            Evaluator.ReferenceAssembly(typeof(ScriptDriver).Assembly);

            //Using evaluator to run some code
            //via our Extension method
            @"using System;
              using System.Linq;
              using MonoHost;
              using System.Collections.Generic;"
                .Run();

            //Step 2 - Adding few items to the Context
            Context.Items.Add("jim", new Player { Name = "Jim" });
            Context.Items.Add("joe", new Player { Name = "Joe" });

            //Step 3 - Retreive the items from the Context in Evaluator
            @"var jim=(Player)Context.Items['jim'];
              var joe=(Player)Context.Items['joe'];"
                 .Run(true);

            Console.WriteLine("<<-- Game Started - Control Jim and Joe. -->>");

            while (true)
            {
                Console.Write("->");
                string input = Console.ReadLine();
                if (input.Equals("@@")) return;
                try { input.Run(); }
                catch (Exception ex) { Console.WriteLine(ex.Message); }
            }
        }
    }

}

There are a couple of simple, but interesting tricks to note in the above code. We have a Context class with a static dictionary inside the same, to pass data back and forth between our main application and the Evaluator.

  • In Step 1, see how are are adding the current assembly as a reference to the Evaluator, so that we can access the Context class.
  • In Step 2, we are filling the context with some instances created in the main application
  • In Step 3, see how we are retrieving the instances in the Evaluator, from the context

Now, as we have our objects exposed to Evaluator, we can use them from the script. Run the application, and have fun with invoking methods of your ‘jim’ and ‘joe’ objects in the script. This is my script console when I run the above program

<<-- Game Started - Control Jim and Joe. Enter @@ to stop -->>
->jim.Attack(joe);
Jim is attacking Joe
->for (int i=0;i<40;i+=5) joe.Goto(20,30+i);
Joe is going to 20,30
Joe is going to 20,35
Joe is going to 20,40
Joe is going to 20,45
Joe is going to 20,50
Joe is going to 20,55
Joe is going to 20,60
Joe is going to 20,65
->joe.Jump();
Joe is jumping
->jim.Jump();
Jim is jumping
->joe.Attack(jim);
Joe is attacking Jim
->

That is cool, isn’t it? So now you know how to expose your ‘mainstream’ objects to Evaluator, for providing some C# scripting features in your application. Happy Coding!!

 

Also, don’t miss my earlier post 4 .NET Libraries You *Should* know better

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