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

/// <summary>
///Evaluator 的摘要说明
/// </summary>
/// 
namespace ItDeer.Public
{
    public class Evaluator
    {
        object _compiled = null;

private string m_formula;

///   <summary>   
        ///   计算公式   
        ///   </summary>   
        public string Formula
        {
            get
            {
                return m_formula;
            }
            set
            {
                m_formula = value;
            }
        }

public Evaluator()
        {
        }

public Evaluator(string formula)
        {
            Formula = formula;
        }

public Double Execute()
        {
            if (Formula == null || Formula == "")
            {
                throw new Exception("请先设置Formula属性!");
            }
            return this.Execute(Formula);
        }
        public Double Execute(string formula)
        {
            constructEvaluator(formula);
            MethodInfo m = _compiled.GetType().GetMethod("GetValue");
            return (Double)m.Invoke(_compiled, null);
        }
        private void constructEvaluator(string formula)
        {
            ICodeCompiler compiler = (new CSharpCodeProvider().CreateCompiler());
            CompilerParameters cp = new CompilerParameters();
            cp.ReferencedAssemblies.Add("system.dll");

cp.GenerateExecutable = false;
            cp.GenerateInMemory = true;

StringBuilder str = new StringBuilder();
            str.Append("using   System;   \n");
            str.Append("namespace   Stoway   {   \n");
            str.Append("public   class   Formula   {   \n");

str.AppendFormat(" public   {0}   GetValue()", "Double");
            str.Append("{");
            str.AppendFormat(" return   Convert.ToDouble({0});   ", formula);
            str.Append("}\n");
            str.Append("}\n");
            str.Append("}");

CompilerResults cr = compiler.CompileAssemblyFromSource(cp, str.ToString());
            if (cr.Errors.HasErrors)
            {
                throw new Exception("不是正确的表达式");
            }
            Assembly a = cr.CompiledAssembly;
            _compiled = a.CreateInstance("Stoway.Formula");
        }
        public static Double GetValue(string formula)
        {
            return new Evaluator().Execute(formula);
        }
    }
}