C#-powered JavaScript Expression Evaluator

Here’s an interesting little class that I initially created to do dynamic arithmetic.  However, after getting the concept up and running, it turns out to be a lot more powerful.  The idea is to start up a hosted JavaScript runtime inside the CLR runtime and present it with string of JavaScript code for immediate inline execution.  The ExpressionEvaluator dutifully loads and evaluates the “scriptlet”, and then it returns the results back to .NET as a string, which can easily be coerced back into an instance of a native CLR Type.  In the original case, this allowed for a javascript-powered dynamic calculator from the statically typed C#.NET, VB.NET, or F# language runtimes.  However, there are more interested use cases for this technology, beyond what I demonstrate here, such as providing a user extensibility layer to the logic of a compiled application at runtime.  For example, allowing users to generate, extend, and mutate existing business rules could easily be achieved by allowing users to introduce script text directly into a captive execution sandbox.  User-specified business rules could be stored as simple text and spooled for execution in workflow engine’s, etc.  Another interesting use case, might be to hook up a REPL (Read Eval Print Loop) into a UI and allow users to issue any sort structured arithmetic expressions directly to the engine, having it respond back with answers in realtime.

Here is a simple unit test showing how to drive it:

using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace Unit.Core
{
 [TestClass]
 public class ExpressionEvaluatorTest
 {
 public TestContext TestContext { get; set; }

[TestMethod]
 public void EvaluateToStringTest()
 {
 var result = ExpressionEvaluator.EvaluateToString("(2+5) < 8");
 Assert.IsTrue(Convert.ToBoolean(result));
 }
 }
}

Here is the ExpressionEvaluator:

using System;
using System.CodeDom.Compiler;
using System.Globalization;
using System.Reflection;
using Microsoft.JScript;

namespace Core.Utils
{
 public static class ExpressionEvaluator
 {
 #region static members
 private static object _evaluator = GetEvaluator();
 private static Type _evaluatorType;
 private const string _evaluatorSourceCode =
 @"package Evaluator
 {
 class Evaluator
 {
 public function Eval(expr : String) : String
 {
 return eval(expr);
 }
 }
 }";

#endregion

#region static methods
 private static object GetEvaluator()
 {
 CompilerParameters parameters;
 parameters = new CompilerParameters();
 parameters.GenerateInMemory = true;

JScriptCodeProvider jp = new JScriptCodeProvider();
 CompilerResults results = jp.CompileAssemblyFromSource(parameters, _evaluatorSourceCode);

Assembly assembly = results.CompiledAssembly;
 _evaluatorType = assembly.GetType("Evaluator.Evaluator");

return Activator.CreateInstance(_evaluatorType);
 }

/// <summary>
 /// Executes the passed JScript Statement and returns the string representation of the result
 /// </summary>
 /// <param name="statement">A JScript statement to execute</param>
 /// <returns>The string representation of the result of evaluating the passed statement</returns>
 public static string EvaluateToString(string statement)
 {
 object o = EvaluateToObject(statement);
 return o.ToString();
 }

/// <summary>
 /// Executes the passed JScript Statement and returns the result
 /// </summary>
 /// <param name="statement">A JScript statement to execute</param>
 /// <returns>The result of evaluating the passed statement</returns>
 public static object EvaluateToObject(string statement)
 {
 lock (_evaluator)
 {
 return _evaluatorType.InvokeMember(
 "Eval",
 BindingFlags.InvokeMethod,
 null,
 _evaluator,
 new object[] { statement },
 CultureInfo.CurrentCulture
 );
 }
 }
 #endregion
 }
}

2 thoughts on “C#-powered JavaScript Expression Evaluator”

  1. Hi All,

    If you want to evaluate a string expression use the below code snippet.

    using System.Data;
     
    DataTable dt = new DataTable();
    var v = dt.Compute("3 * (2+4)",""); 

    Thanks & Regards,
    Ramesh Sambari
    (sambariramesh@gmail.com)

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