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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;

///
/// Implements JScript/JavaScript Eval like
/// functionality in C# to allow dynamic evaluation
/// of expressions.
///
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

    ///
    /// Creates a dynamic in-memory assembly using JScript.NET
    /// to evaluate expressions.
    ///
    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

    ///
    /// Evaluates specified expression using reflection
    /// on the assembly generated in Initialize() method.
    ///
    /// The expression.
    /// A string representing evaluated expression.
    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"));

Some of the features of Anthem.NET include:

• Free and open source
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• Seamless integration with Visual Studio 2005
• Broad browser support (IE, Firefox, and Safari)
• Support for Mono
• Familiar ASP.NET postback style functionality
• Support for ASP.NET ViewState
• Support for web user controls

You can download Anthem.NET from SourceForge.