Dynamic Filtering and Querying in Your .NET applications - Using Mono’s C# Compiler As Service

By Anoop Madhusudanan

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imageIn my last post, I explained how to host Mono’s C# Compiler As A Service in your .NET applications. In this post, we’ll explore this a bit further, and see how to leverage Mono’s compiler as a service for implementing some dynamic querying features in your .NET applications.

Read my first post How To Host Mono’s C# Compiler as a Service in your .NET Application if you missed it. Now, let us explore some more points about using Mono’s C# Compiler as a Service in general, and the ‘Evaluator’ class in particular, to see how to leverage this for some practical uses like dynamic data filtering.

The Mono.CSharp.Evaluator Class

Let us consider a simple and dirty example. Fire up a Console application in Visual Studio, Add a reference to Mono.CSharp.dll, and try this code. Have a look at the Init method, and Run method of the Mono.CSharp.Evaluator class

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

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

            //Using evaluator to run some code, line by line

            //Importing namespaces 
            Evaluator.Run("using System;");
            Evaluator.Run("using System.Linq;");
            Evaluator.Run("using System.Collections.Generic;");

            //Sum of 'n' numbers
            Evaluator.Run("List<int> numbers= new List<int> {1,2,4,3} ;");
            Evaluator.Run("var sum=0; foreach(var num in numbers) sum+=num;");
            Evaluator.Run("Console.WriteLine(sum);");

        }
    }
}

Sweetening the Run

Of course, we can make the above code sweeter with an extension method. Have a look at this, the above code re-written. Nothing much, just some syntax sweetness.

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

namespace MonoHost
{
    // A simple extension method to invoke Evaluator.Run on a string
    public static class EvaluatorExtensions
    {
        public static void Run(this string code, bool repQuotes = false)
        {
            var run = repQuotes ? 
                      code.Replace("'", "\"") : code;
            Evaluator.Run(run);
        }
    }

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

            //Importing namespaces. Note that @”..” is one single string in multiple lines
            @"using System;
              using System.Linq;
              using System.Collections.Generic;"
                .Run();

            //Sum of 'n' numbers
            @"List<int> numbers= new List<int> {1,2,4,3} ;
             var sum=0; 
             foreach(var num in numbers) 
                     sum+=num;
             Console.WriteLine('Result={0}',sum);"
                .Run(true);
        }
    }
}

The Evaluate method

The Evaluator class also has an Evaluate method, that can evaluate an expression and can pass something back to your .NET code. This is pretty interesting, as this allows you to build dynamic queries, predicates etc via plain text, and apply it to your native collections etc. What about creating another LinqPad, your own style? Check out this example, where we query a list of integers by building a dynamic predicate.

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

namespace MonoHost
{
    // A simple extension method 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);
        }
    }

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

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

            List<int> numbers = new List<int> { 1, 30, 90, 23, 46, 22, 50 };

            //Filter the numbers. Dynamic Predicate
            var match=@"new Predicate<int>(num=> num>20 && num <44);".Eval<Predicate<int>>();
            var filtered = numbers.FindAll(match);
            filtered.ForEach(n => Console.WriteLine(n));
        }
    }
}

The interesting point to note in the above example is, we are building our Predicate based on a string. And our Eval extension method is good enough to cast the result to the specified type. Now, there are a handful use cases for this. For example, with a bit of creativity and some pre-processing, you can provide a user friendly query language against data in your view model, to filter records when you build your next data visualizer application. And as an exercise, try modifying the above example with a custom type list, instead of an integer list. You can add a reference to an assembly via the ReferenceAssembly method of the Evaluator class.

Conclusion

Now, it is pretty cool if the context and data can be shared across the main application and the Evaluator context, so that our main app objects and data can be ‘touched’ from the plain text code that’ll be executed by the Evaluator, right? So that we can implement some sort of Scripting functionality via C#!! There are various ways to do that, but that is the topic for my next post. So stay tuned!!

Updated: I blogged the Next Part  - Using C# As A Scripting Language in your .NET Applications

Read some of my previous C# posts – Revisiting Few C# Concepts or 7 Free Ebooks for .NET programmers and Architects etc.

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