表 达式树

》》》可以借助 LINQPad工具

using System;
using System.Collections.Generic;
using System.Data.Entity;
using System.Linq;
using System.Linq.Expressions;
using System.Text;
using System.Threading.Tasks;
using System.Transactions;

namespace EFDemo
{
    public class Product
    {
        public string Product_Name { get; set; }
        public int Product_Price { get; set; }
    }
    class Program
    {
        static void Main(string[] args)
        {
            List<Product> prolist = new List<Product>() {
                new Product(){ Product_Name="苹果手机",Product_Price=5999},
                new Product(){ Product_Name="华为手机",Product_Price=4999},
                new Product(){ Product_Name="华为手机",Product_Price=6999},
                new Product(){ Product_Name="苹果手机",Product_Price=9999}
            };
            var result1= prolist.Where(c =>{ return c.Product_Name == "苹果手机" && c.Product_Price > 6000; });
            foreach (var r in result1)
            {
                Console.WriteLine($"产品名称：{r.Product_Name} 价格：{r.Product_Price}");
            }
            var item=Expression.Parameter(typeof(Product), "item");
            var item_name = Expression.Property(item, "Product_Name");
            var item_price= Expression.Property(item, "Product_Price");
            var query_Name = Expression.Constant("苹果手机");
            var query_Price = Expression.Constant(6000);
            var c1 = Expression.Equal(item_name, query_Name);
            var c2 = Expression.GreaterThan(item_price, query_Price);
            var cc = Expression.And(c1, c2);
            var lambda_expression = Expression.Lambda<Func<Product, bool>>(cc, item);
            var reuslt = lambda_expression.Compile();
           
            Console.WriteLine(lambda_expression.ToString());
            var res = prolist.Where(reuslt);
            foreach (var r in res)
            {
                Console.WriteLine($"产品名称：{r.Product_Name} 价格：{r.Product_Price}");
            }
            Console.ReadKey();
        }
    }
}

》》》表达式树 编译之后 才是委托

利用表达式树 对象的映射

》》》定义一个泛型静态类 ExpressionMapper<Tin ,Tout>

using System;
using System.Collections.Generic;
using System.Linq;
using System.Linq.Expressions;
using System.Text;
using System.Threading.Tasks;

namespace EFDemo
{
     public class ExpressionMapper<Tin ,Tout>
    {
        private static Func<Tin, Tout> _Func = null;
        static ExpressionMapper()
        {
            ParameterExpression paramExp = Expression.Parameter(typeof(Tin), "p");
            List<MemberBinding> memberBindingList = new List<MemberBinding>();
            //绑定属性
            foreach (var item in typeof(Tout).GetProperties())
            {
                MemberExpression member = Expression.Property(paramExp, typeof(Tin).GetProperty(item.Name));
                MemberBinding memberBinding = Expression.Bind(item,member);
                memberBindingList.Add(memberBinding);
            }
            //绑定字段
            foreach (var item in typeof(Tout).GetFields())
            {
                MemberExpression member = Expression.Field(paramExp, typeof(Tin).GetField(item.Name));
                MemberBinding memberBinding = Expression.Bind(item, member);
                memberBindingList.Add(memberBinding);
            }
            //创建新对象并初始化
            MemberInitExpression memberInitExp = Expression.MemberInit(Expression.New(typeof(Tout)),memberBindingList.ToArray());
            Expression<Func<Tin, Tout>> funcExp = Expression.Lambda<Func<Tin, Tout>>(memberInitExp, new ParameterExpression[] { paramExp });
            _Func = funcExp.Compile();
        }

        public static Tout Mapper(Tin t)
        {
            return _Func(t);
        }
    }
}