动态创建表达式树

表达式树是一种数据结构，以数据形式表示语言级代码。.Net平台引入的"逻辑即数据"概念。

所有的数据都存储在树结构中，每个结点表示一个表达式(Expression)。要想手动生成表达式树我们需要引用System.Linq.Expressions 命名空间,最重要的一个类是Expression,它是所有表达式的基类。

using System;using System.Collections.Generic;using System.Linq;using System.Linq.Expressions;using System.Text;using System.Reflection;namespace fofo.ExpressionTree{    public class ExpressionWhere    {        private class UserInfo        {            public string Name { get; set; }            public int Age { get; set; }        }        ///        /// build: info => ((info.Name.Contains("en")) And info.Age > 10 And info.Age < 20)        ///        private Expression<Func<UserInfo, bool>> BuildExpressionTree()        {            // condition Name Contains "en" and Age > 10 and Age < 20            Expression where;            // parameter UserInfo            ParameterExpression par = Expression.Parameter(typeof(UserInfo), "info");            // get property (Name)            MemberExpression memName = Expression.Property(par, "Name");            MethodInfo methodContains = typeof(string).GetMethod("Contains");            ConstantExpression constExpress = Expression.Constant("en", typeof(string));            MethodCallExpression callExpress = Expression.Call(memName, methodContains, constExpress);            where = callExpress;            // get property (Age)            MemberExpression memAge = Expression.Property(par, "Age");            ConstantExpression age1 = Expression.Constant(10, typeof(int));            BinaryExpression greaterThan = Expression.GreaterThan(memAge, age1);            where = Expression.And(where, greaterThan);            ConstantExpression age2 = Expression.Constant(20, typeof(int));            BinaryExpression lessThan = Expression.LessThan(memAge, age2);            where = Expression.And(where, lessThan);            Expression<Func<UserInfo, bool>> lambda = Expression.Lambda<Func<UserInfo, bool>>(where, par);            //Func<UserInfo, bool> func = lambda.Compile();            return lambda;        }        public string TestBuildExpressionTree()        {            Expression<Func<UserInfo, bool>> lambda = BuildExpressionTree();            string strRet = lambda.ToString() + "\r\n";            Func<UserInfo, bool> func = lambda.Compile();            List<UserInfo> listUser = new List<UserInfo>();            for (int i = 5; i < 25; i++)            {                listUser.Add(new UserInfo { Name = "leng", Age = i });            }            IEnumerable<UserInfo> listTmp = listUser.Where<UserInfo>(func);            foreach (UserInfo info in listTmp)            {                strRet += info.Age.ToString() + "\r\n";            }            return strRet;        }    }}