表达式树

》》》可以借助 LINQPad工具
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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();}}
}

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》》》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);}}
}