表达式树基础代码

using System;

using System.Collections.Generic;

using System.Linq;

using System.Reflection;

using System.Text;

using System.Threading.Tasks;

using System.Linq.Expressions;



namespace TestExpressionLambda

{

    class ProgramExpression

    {

        static void Main(string[] args)

        {

            ExpressionAdd();



            ExpressionFromlambda();



            ExpressionStartWith();



            Console.Read();

        }



        /// <summary>

        /// 最简单的加法表达式树

        /// 

        ///                   根节点

        ///                   BanaryExpression

        ///                   NodeType = Add

        ///                   Type=System.Int32

        /// 

        ///     左叶子                          右叶子

        ///     firstArg                        secondArg

        ///     ConstantExpression              ConstantExpression

        ///     NodeType=Constant               NodeType=Constant

        ///     Type=System.Int32               Type=System.Int32

        ///     Value=3                         Value=2

        /// 

        /// 

        /// </summary>

        private static void ExpressionAdd()

        {

            //构建常量表达式3

            ConstantExpression firstArg = Expression.Constant(3);

            //构建常量表达式2

            ConstantExpression secondArg = Expression.Constant(2);



            //构建+表达式 3+2

            BinaryExpression add = Expression.Add(firstArg, secondArg);

            Console.WriteLine(add);//输出 : (3+2)



            //获取委托表达式

            Expression<Func<int>> addExpression = Expression.Lambda<Func<int>>(add);

            //Compile出委托，内部调用了emit去构造il

            Func<int> addFunc = addExpression.Compile();



            Console.WriteLine(addFunc);//输出System.Func~1[Int32]

            Console.WriteLine(addFunc());//输出执行的结果：5

        }



        /// <summary>

        /// 简单从lambda创建表达式树

        /// ps:不能讲带有语句块的lambda转换成表达式树

        /// </summary>

        private static void ExpressionFromlambda()

        {

            Expression<Func<int>> lambadExpression = () => 3 + 2;

            Func<int> lambdaFunc = lambadExpression.Compile();

            Console.WriteLine(lambdaFunc);

            Console.WriteLine(lambdaFunc());

        }



        /// <summary>

        /// 表达式树去调用string类型的StartWith方法

        /// 要实现类型lambda表达式：(x,y) => x.StartsWith(y)

        /// </summary>

        private static void ExpressionStartWith()

        {

            //反射取到string的StartsWith方法

            MethodInfo method = typeof(string).GetMethod("StartsWith", new[] { typeof(string) });



            //取参数表达式:x,y

            ParameterExpression x = Expression.Parameter(typeof(string), "x");

            ParameterExpression y = Expression.Parameter(typeof(string), "y");



            //StartsWith方法需要的参数

            Expression[] methodArgs = new[] { y };

            //生成调用表达式

            MethodCallExpression call = Expression.Call(x, method, y);



            var lambdaParameters = new[] { x, y };

            //Expression.Lambda方法生成表达式树委托

            Expression<Func<string, string, bool>> lambda = Expression.Lambda<Func<string, string, bool>>(call,

                lambdaParameters);



            //Compile出委托

            Func<string, string, bool> func = lambda.Compile();



            Console.WriteLine(func("First", "Second"));

            Console.WriteLine(func("First", "Fir"));

        }

    }

}