如果在文本框中输入简单的数学计算式：200+100*290之类的计算式，如何计算出他们的值:

using System;
using System.CodeDom;
using System.CodeDom.Compiler;
using Microsoft.CSharp;
using System.Text;
using System.Reflection;

namespace ADOGuy
{
    /// <summary>
    /// Summary description for Class1.
    /// </summary>
    public class Evaluator
    {

        public Evaluator(EvaluatorItem[] items)
        {
            ConstructEvaluator(items);
        }

        public Evaluator(Type returnType, string expression, string name)
        {
            EvaluatorItem[] items = { new EvaluatorItem(returnType, expression, name) };
            ConstructEvaluator(items);
        }

        public Evaluator(EvaluatorItem item)
        {
            EvaluatorItem[] items = { item };
            ConstructEvaluator(items);
        }

        private void ConstructEvaluator(EvaluatorItem[] items)
        {
            ICodeCompiler comp = (new CSharpCodeProvider().CreateCompiler());
            CompilerParameters cp = new CompilerParameters();
            cp.ReferencedAssemblies.Add("system.dll");
            cp.ReferencedAssemblies.Add("system.data.dll");
            cp.ReferencedAssemblies.Add("system.xml.dll");
            cp.GenerateExecutable = false;
            cp.GenerateInMemory = true;

            StringBuilder code = new StringBuilder();
            code.Append("using System; \n");
            code.Append("using System.Data; \n");
            code.Append("using System.Data.SqlClient; \n");
            code.Append("using System.Data.OleDb; \n");
            code.Append("using System.Xml; \n");
            code.Append("namespace ADOGuy { \n");
            code.Append("  public class _Evaluator { \n");
            foreach(EvaluatorItem item in items)
            {
                code.AppendFormat("