JScript Eval Method In C#

JavaScript and JScript languages provide a convenient Eval method that allows for dynamic evaluation of expressions. This functionality can be duplicated in C# using Microsoft's JScript CodeDomProvider and Reflection. Following class automatically compiles an in-memory assembly using JScript CodeDomProvider. The Eval method uses this assembly to dynamically evaluate specified expressions: using System; using System.CodeDom.Compiler; using System.Reflection; using Microsoft.JScript; /// <summary> /// Implements JScript/JavaScript Eval like /// functionality in C# to allow dynamic evaluation /// of expressions. /// </summary> public static class ExpressionEvaluator { #region Private Members private static Type _evaluatorType; private static object _evaluatorInstance; private static readonly string _jscriptEvalClass = @"import System; class JScriptEvaluator { public static function Eval(expression : String) : String { return eval(expression); } }"; #endregion #region Private Methods /// <summary> /// Creates a dynamic in-memory assembly using JScript.NET /// to evaluate expressions. /// </summary> private static void Initialize() { CodeDomProvider compiler = new JScriptCodeProvider(); CompilerParameters parameters = new CompilerParameters(); parameters.GenerateInMemory = true; parameters.ReferencedAssemblies.Add("system.dll"); CompilerResults results = compiler.CompileAssemblyFromSource(parameters, _jscriptEvalClass); Assembly assembly = results.CompiledAssembly; _evaluatorType = assembly.GetType("JScriptEvaluator"); _evaluatorInstance = Activator.CreateInstance(_evaluatorType); } #endregion #region Public Methods /// <summary> /// Evaluates specified expression using reflection /// on the assembly generated in Initialize() method. /// </summary> /// <param name="expression">The expression.</param> /// <returns>A string representing evaluated expression.</returns> public static string Eval(string expression) { if (_evaluatorInstance == null) Initialize(); object result = _evaluatorType.InvokeMember( "Eval", BindingFlags.InvokeMethod, null, _evaluatorInstance, new object[] { expression } ); return (result == null) ? null : result.ToString(); } #endregion }

Using this class, we can execute statements such as:

Console.WriteLine(ExpressionEvaluator.Eval("2 + 9")); Console.WriteLine(ExpressionEvaluator.Eval("System.DateTime")); Console.WriteLine(ExpressionEvaluator.Eval("Environment.OSVersion.VersionString"));