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C#知识点-表达式目录树

原文链接：https://www.cnblogs.com/loverwangshan/p/10254730.html

阅读目录

一：什么是表达式树

Expression我们称为是表达式树，是一种数据结构体，用于存储需要计算，运算的一种结构，这种结构可以只是存储，而不进行运算。通常表达式目录树是配合Lambda一起来使用的，lambda可以是匿名方法，当然也可以使用Expression来动态的创建！下面我们举例来说明什么是表达式目录树。

先创建一个People的实体，下面会用到

/// <summary>
/// 实体类
/// </summary>
public class People
{
    public int Age { get; set; }
    public string Name { get; set; }
    public int Id;
}

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我们可以通过下面创建表达式目录树，我们称之为A种方式：

Expression<Func<People, bool>> lambda = x => x.Id.ToString().IndexOf("5") >= 0;

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我们还可以使用Expression来动态创建，我们称之为B种方式：

var peopleParam = Expression.Parameter(typeof(People), "x");//创建一个x，类型为people
//得到x.Id
MemberExpression idParam = Expression.Field(peopleParam, "Id");

//得到ToString方法
MethodInfo toStringWay = typeof(int).GetMethod("ToString", new Type[] { });

//得到IndexOf的方法，然后new Type[]这个代表是得到参数为string的一个方法
MethodInfo indexOfWay = typeof(string).GetMethod("IndexOf", new Type[] { typeof(string) });

//通过下面方法得到x.Id.ToString()
MethodCallExpression tostringResult = Expression.Call(idParam, toStringWay, new Expression[] { });

//通过下面方法得到x.Id.ToString().IndexOf("5") ,MethodCallExpression继承于Expression
MethodCallExpression indexOfResult = Expression.Call(tostringResult, indexOfWay, new Expression[] { Expression.Constant("5") });

//x.Id.ToString().IndexOf("5")>=0
var lambdaBody = Expression.GreaterThanOrEqual(indexOfResult, Expression.Constant(0));

//得到x => x.Id.ToString().IndexOf("5") >= 0，后面的一个参数指的是x，如果有多个则指定多个
Expression<Func<People,bool>> lambdaResult = Expression.Lambda<Func<People, bool>>(lambdaBody, new ParameterExpression[]
                                                                                                { peopleParam });

//通过lambdaResult.Compile()得到Func<People,bool>这样的委托，然后Invoke是调用委托
bool result = lambdaResult.Compile().Invoke(new People() { Id = 155 });

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A种和B种得到的结果是一致的，只不过第一种是通过lambda匿名方法来构建，第二种是通过动态的Expression来构建。另外下面的原理也是一样的

//普通的Lambda表达式
 Func<int,int,int> func = (x,y)=>  x + y - 2;
//表达式目录树的Lambda表达式声明方式
Expression<Func<int, int, int>> expression = (x, y) => x + y - 2;   

//表达式目录树的拼接方式实现
ParameterExpression parameterx =  Expression.Parameter(typeof(int), "x");
ParameterExpression parametery =  Expression.Parameter(typeof(int), "y");
ConstantExpression constantExpression = Expression.Constant(2, typeof(int));
BinaryExpression binaryAdd = Expression.Add(parameterx, parametery);
BinaryExpression binarySubtract = Expression.Subtract(binaryAdd, constantExpression);
Expression<Func<int, int, int>> expressionMosaic = Expression.Lambda<Func<int, int, int>>(binarySubtract, new ParameterExpression[]
{
       parameterx,
       parametery
});

int ResultLambda = func(5, 2);
int ResultExpression = expression.Compile()(5, 2);
int ResultMosaic = expressionMosaic.Compile()(5, 2);
Console.WriteLine($"func:{ResultLambda}");
Console.WriteLine($"expression:{ResultExpression}");
Console.WriteLine($"expressionMosaic:{ResultMosaic}");

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下面举例说明以下Expression.Block

ParameterExpression varExpr = Expression.Variable(typeof(int), "x"); //add(int x);
var ex1 = Expression.Assign(varExpr, Expression.Constant(1)); //x = 1; var ex1 = x;
var ex2 = Expression.Add(ex1, Expression.Constant(5)); //var ex2 = ex1 + 5;//6
var ex4 = Expression.Add(ex2, Expression.Constant(9)); //var ex4 = ex2 + 9; //15
var ex5 = Expression.Add(ex4, Expression.Constant(8)); // var ex5 = ex4 + 8; //23
BlockExpression blockExpr = Expression.Block(
    new ParameterExpression[] { varExpr },
    ex1,
    ex2,
    ex4,
    ex5
);

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该代码等效于，返回的结果都以最后一个Expression为主，则为ex5这个表达式

public int add(int x)
{
    x = 1;
    var ex1 = x;
    var ex2 = ex1 + 5;//6
    var ex4 = ex2 + 9; //15
    var ex5 = ex4 + 8; //23
    return ex5; //23
}

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Expression.Block没有返回值

{   
    Expression A = Expression.Constant("第一大");
    Expression B = Expression.Constant("第二大");
    Expression ex = Expression.GreaterThan(Expression.Constant(1), Expression.Constant(2));

    var method = typeof(Console).GetMethod("WriteLine", new Type[] { typeof(string) });
    var AM = Expression.Call(method, A);
    var BM = Expression.Call(method, B);

    var condition = Expression.IfThenElse(ex, AM, BM);
    var blockExpr = Expression.Block(condition); //IfThenElse是没有返回值的

    foreach (var expr in blockExpr.Expressions)
        Console.WriteLine(expr.ToString());

    var lambdaExpression = Expression.Lambda<Action>(blockExpr).Compile();
    lambdaExpression();
}

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下图是Expression的一些变量

二：表达式目录树与委托

Expression一般都是都是配合委托一起来使用的，比如和委托Action（没有返回值）,Func（至少有一个返回参数，且最后一个值为返回参数），Action，Func既可以直接传入一个与之匹配的实体方法，又可以传入lambda表达式这种匿名类（这种是声明lambda表达式的一种快捷方式）。Expression，Action,Func关键词是在.net 3.5之后出现的。Expression<Func<>>是可以转成Func的(通过compile()这个方法转换)。反过来则不行。我们可以理解为Func<>经过定义后，就无法改变它了。而表达式树(Expression<Func<>>则是可以进行变更的。Lambda使用lambda表达声明表达式目录树的时候注意不能有｛｝，即：

Func<int, int, int> func = (m, n) => m * n + 2;

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上面这样是可以的。但是下面这样是不被允许的：

 Expression<Func<int, int, int>> exp1 = (m, n) =>
  {
          return m * n + 2;
  };//不能有语句体   只能是一行，不能有大括号

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下面的例子来解析一下委托和表达式目录树

#region PrivateMethod
 private static void Do1(Func<People, bool> func)
 {
     List<People> people = new List<People>();
     people.Where(func);
 }
 private static void Do1(Expression<Func<People, bool>> func)
 {
     List<People> people = new List<People>()
     {
         new People(){Id=4,Name="123",Age=4},
         new People(){Id=5,Name="234",Age=5},
         new People(){Id=6,Name="345",Age=6},
     };

     List<People> peopleList = people.Where(func.Compile()).ToList();
 }

 private static IQueryable<People> GetQueryable(Expression<Func<People, bool>> func)
 {
     List<People> people = new List<People>()
     {
         new People(){Id=4,Name="123",Age=4},
         new People(){Id=5,Name="234",Age=5},
         new People(){Id=6,Name="345",Age=6},
     };

     return people.AsQueryable<People>().Where(func);
 }
 #endregion

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然后调用的时候为如下：

Expression<Func<People, bool>> lambda1 = x => x.Age > 5;
Expression<Func<People, bool>> lambda2 = x => x.Id > 5;
Expression<Func<People, bool>> lambda3 = lambda1.And(lambda2);
Expression<Func<People, bool>> lambda4 = lambda1.Or(lambda2);
Expression<Func<People, bool>> lambda5 = lambda1.Not();
Do1(lambda3);
Do1(lambda4);
Do1(lambda5);

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三：使用Expression来进行不同对象的相同名字的属性映射

如果我们有一个新的对象和People属性基本上一致，如下：

/// <summary>
/// 实体类Target
/// PeopleDTO
/// </summary>
public class PeopleCopy
{

    public int Age { get; set; }
    public string Name { get; set; }
    public int Id;
}

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现在我们想要把People的中Age，Name，Id等赋值给PeopleCopy，第一种我们直接想到的是硬编码，然后如下：

People people = new People()
{
    Id = 11,
    Name = "加菲猫",
    Age = 31
};
//PeopleCopy copy = (PeopleCopy)people; //这种强制转换肯定是不行的

PeopleCopy peopleCopy = new PeopleCopy()
{
    Id = people.Id,
    Name = people.Name,
    Age = people.Age
};

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但是如果有多个类型转换，要写N次，然后不同用且费力，所以我们会想到通用的方法，比如使用：【反射】，【序列化反序列化】，【缓存+表达式目录】，【泛型+表达式目录】，【AutoMapper】，我们可以用这五种方法都小试一下！

1：反射完成对象属性映射

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

namespace ExpressionDemo.MappingExtend
{
    public class ReflectionMapper
    {
        /// <summary>
        /// 反射
        /// </summary>
        /// <typeparam name="TIn"></typeparam>
        /// <typeparam name="TOut"></typeparam>
        /// <param name="tIn"></param>
        /// <returns></returns>
        public static TOut Trans<TIn, TOut>(TIn tIn)
        {
            TOut tOut = Activator.CreateInstance<TOut>();
            foreach (var itemOut in tOut.GetType().GetProperties())
            {
                var propIn = tIn.GetType().GetProperty(itemOut.Name);
                itemOut.SetValue(tOut, propIn.GetValue(tIn));
            }
            foreach (var itemOut in tOut.GetType().GetFields())
            {
                var fieldIn = tIn.GetType().GetField(itemOut.Name);
                itemOut.SetValue(tOut, fieldIn.GetValue(tIn));
            }
            return tOut;
        }
    }
}

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2：使用序列化和反序列化来完成对象属性映射：

using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ExpressionDemo.MappingExtend
{
    /// <summary>
    /// 使用第三方序列化反序列化工具
    /// 
    /// 还有automapper
    /// </summary>
    public class SerializeMapper
    {
        /// <summary>
        /// 序列化反序列化方式
        /// </summary>
        /// <typeparam name="TIn"></typeparam>
        /// <typeparam name="TOut"></typeparam>
        public static TOut Trans<TIn, TOut>(TIn tIn)
        {
            return JsonConvert.DeserializeObject<TOut>(JsonConvert.SerializeObject(tIn));
        }
    }
}

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3：缓存+表达式目录树

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

namespace ExpressionDemo.MappingExtend
{
    /// <summary>
    /// 生成表达式目录树 缓存
    /// </summary>
    public class ExpressionMapper
    {
        /// <summary>
        /// 字典缓存--hash分布
        /// </summary>
        private static Dictionary<string, object> _Dic = new Dictionary<string, object>();

        /// <summary>
        /// 字典缓存表达式树
        /// </summary>
        /// <typeparam name="TIn"></typeparam>
        /// <typeparam name="TOut"></typeparam>
        /// <param name="tIn"></param>
        /// <returns></returns>
        public static TOut Trans<TIn, TOut>(TIn tIn)
        {
            string key = string.Format("funckey_{0}_{1}", typeof(TIn).FullName, typeof(TOut).FullName);
            if (!_Dic.ContainsKey(key))
            {
                ParameterExpression parameterExpression = Expression.Parameter(typeof(TIn), "p");
                List<MemberBinding> memberBindingList = new List<MemberBinding>();
                foreach (var item in typeof(TOut).GetProperties())
                {
                    MemberExpression property = Expression.Property(parameterExpression, typeof(TIn).GetProperty(item.Name));
                    MemberBinding memberBinding = Expression.Bind(item, property);
                    memberBindingList.Add(memberBinding);
                }
                foreach (var item in typeof(TOut).GetFields())
                {
                    MemberExpression property = Expression.Field(parameterExpression, typeof(TIn).GetField(item.Name));
                    MemberBinding memberBinding = Expression.Bind(item, property);
                    memberBindingList.Add(memberBinding);
                }
                MemberInitExpression memberInitExpression = Expression.MemberInit(Expression.New(typeof(TOut)), memberBindingList.ToArray());
                Expression<Func<TIn, TOut>> lambda = Expression.Lambda<Func<TIn, TOut>>(memberInitExpression, new ParameterExpression[]
                {
                    parameterExpression
                });
                Func<TIn, TOut> func = lambda.Compile();//拼装是一次性的
                _Dic[key] = func;
            }
            return ((Func<TIn, TOut>)_Dic[key]).Invoke(tIn);
        }
    }
}

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4：泛型+表达式目录树

using System;
using System.Collections.Generic;
using System.Linq.Expressions;

namespace ExpressionDemo.MappingExtend
{
    /// <summary>
    /// 生成表达式目录树  泛型缓存
    /// </summary>
    /// <typeparam name="TIn"></typeparam>
    /// <typeparam name="TOut"></typeparam>
    public class ExpressionGenericMapper<TIn, TOut>//Mapper`2
    {
        private static Func<TIn, TOut> _FUNC = null;
        static ExpressionGenericMapper()
        {
            ParameterExpression parameterExpression = Expression.Parameter(typeof(TIn), "p");
            List<MemberBinding> memberBindingList = new List<MemberBinding>();
            foreach (var item in typeof(TOut).GetProperties())
            {
                MemberExpression property = Expression.Property(parameterExpression, typeof(TIn).GetProperty(item.Name)); //p.Age
                MemberBinding memberBinding = Expression.Bind(item, property); //Age=p.Age
                memberBindingList.Add(memberBinding);
            }
            foreach (var item in typeof(TOut).GetFields())
            {
                MemberExpression property = Expression.Field(parameterExpression, typeof(TIn).GetField(item.Name));
                MemberBinding memberBinding = Expression.Bind(item, property);
                memberBindingList.Add(memberBinding);
            }
            //new PeopleCopy() {Age = p.Age, Name = p.Name, Id = p.Id}
            MemberInitExpression memberInitExpression = Expression.MemberInit(Expression.New(typeof(TOut)), memberBindingList.ToArray());
            //p => new PeopleCopy() {Age = p.Age, Name = p.Name, Id = p.Id}
            Expression<Func<TIn, TOut>> lambda = Expression.Lambda<Func<TIn, TOut>>(memberInitExpression, new ParameterExpression[]
            {
                    parameterExpression
            });
            _FUNC = lambda.Compile();//拼装是一次性的
        }
        public static TOut Trans(TIn t)
        {
            return _FUNC(t);
        }
    }
}

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5：使用.netFramwork框架自带的AutoMapper，首先我们要nuget添加引用AutoMapper即可直接使用，具体代码为：

using AutoMapper;

namespace ExpressionDemo.MappingExtend
{
    public class AutoMapperTest
    {
        public static TOut Trans<TIn, TOut>(TIn tIn)
        {
            return Mapper.Instance.Map<TOut>(tIn);
        }
    }
}

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五种方法我们分别调用一下，然后测试一下性能，代码如下：

{
                People people = new People()
                {
                    Id = 11,
                    Name = "加菲猫",
                    Age = 31
                };
                //使用AutoMapper之前必须要初始化对应的关系
                Mapper.Initialize(x => x.CreateMap<People, PeopleCopy>()); 
               
                long common = 0;
                long generic = 0;
                long cache = 0;
                long reflection = 0;
                long serialize = 0;
                long autoMapper = 0;
                {
                    Stopwatch watch = new Stopwatch();
                    watch.Start();
                    for (int i = 0; i < 1000000; i++)
                    {
                        PeopleCopy peopleCopy = new PeopleCopy()
                        {
                            Id = people.Id,
                            Name = people.Name,
                            Age = people.Age
                        };
                    }
                    watch.Stop();
                    common = watch.ElapsedMilliseconds;
                }
                {
                    Stopwatch watch = new Stopwatch();
                    watch.Start();
                    for (int i = 0; i < 1000000; i++)
                    {
                        PeopleCopy peopleCopy = AutoMapperTest.Trans<People, PeopleCopy>(people);
                    }
                    watch.Stop();
                    autoMapper = watch.ElapsedMilliseconds;
                }
                {
                    Stopwatch watch = new Stopwatch();
                    watch.Start();
                    for (int i = 0; i < 1000000; i++)
                    {
                        PeopleCopy peopleCopy = ReflectionMapper.Trans<People, PeopleCopy>(people);
                    }
                    watch.Stop();
                    reflection = watch.ElapsedMilliseconds;
                }
                {
                    Stopwatch watch = new Stopwatch();
                    watch.Start();
                    for (int i = 0; i < 1000000; i++)
                    {
                        PeopleCopy peopleCopy = SerializeMapper.Trans<People, PeopleCopy>(people);
                    }
                    watch.Stop();
                    serialize = watch.ElapsedMilliseconds;
                }
                {
                    Stopwatch watch = new Stopwatch();
                    watch.Start();
                    for (int i = 0; i < 1000000; i++)
                    {
                        PeopleCopy peopleCopy = ExpressionMapper.Trans<People, PeopleCopy>(people);
                    }
                    watch.Stop();
                    cache = watch.ElapsedMilliseconds;
                }
                {
                    Stopwatch watch = new Stopwatch();
                    watch.Start();
                    for (int i = 0; i < 1000000; i++)
                    {
                        PeopleCopy peopleCopy = ExpressionGenericMapper<People, PeopleCopy>.Trans(people);
                    }
                    watch.Stop();
                    generic = watch.ElapsedMilliseconds;
                }

                Console.WriteLine($"common = { common} ms");
                Console.WriteLine($"reflection = { reflection} ms");
                Console.WriteLine($"serialize = { serialize} ms");
                Console.WriteLine($"cache = { cache} ms");
                Console.WriteLine($"generic = { generic} ms");
                Console.WriteLine($"automapper = { autoMapper} ms");
                //性能比automapper还要高
            }

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运行结果如下：

通过结果发现：反射和序列化运用的时间最多，而我们惊奇的发现表达式目录树+泛型缓存比框架自带的AutoMapper时间还短！有木有感觉超级腻害~！

四：ORM与表达式树目录的关系

我们平常项目中经常用到EF，其实都是继承Queryable，然后我们使用的EF通常都会使用 var items = anserDo.GetAll().Where(x => x.OrganizationId == input.oid || input.oid == 0) ,where其实传的就是表达式目录树。那我们来一步一步解析EF底层实现的具体逻辑。

lambada表达式上面说了能使用Expression来动态拼接，当然它还有一个神奇的功能，能动态的解耦。Expression有个类ExpressionVisitor

这个类中的Visit（Expression node）是解读表达式的入口，然后能够神奇的区分参数和方法体，然后将表达式调度到此类中更专用的访问方法中，然后一层一层的解析下去，一直到最终的叶节点！

将表达式调度到此类中更专用的访问方法中：我们来举例说明：

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

namespace ExpressionDemo.Visitor
{
    public class OperationsVisitor : ExpressionVisitor
    {
        public Expression Modify(Expression expression)
        {
            return this.Visit(expression);
        }

        protected override Expression VisitBinary(BinaryExpression b)
        {
            if (b.NodeType == ExpressionType.Add)
            {
                Expression left = this.Visit(b.Left);
                Expression right = this.Visit(b.Right);
                return Expression.Subtract(left, right);
            }

            return base.VisitBinary(b);
        }

        protected override Expression VisitConstant(ConstantExpression node)
        {
            return base.VisitConstant(node);
        }
    }
}

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下面调用：

1 {
2     //修改表达式目录树
3     Expression<Func<int, int, int>> exp = (m, n) => m * n + 2;
4     OperationsVisitor visitor = new OperationsVisitor();      
5     Expression expNew = visitor.Modify(exp);
6 }

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visit这个这个方法能够识别出来 m*n+2 是个二叉树，会通过下面的图然后一步一步的进行解析，如果遇到m*n 这会直接调用VisitBinary(BinaryExpression b)这个方法，如果遇到m或者n会调用VisitParameter(ParameterExpression node)这个方法，

如果遇到2常量则会调用VisitConstant(ConstantExpression node)，这就是visit神奇的调度功能！

我们EF写的where等lambda表达式，就是通过ExpressionVisitor这个类来反解析的！之前没有学习过表达式目录树，以为ef本来就应该这样写，有没有和我一样认为的？

我们现在模拟写一个lambda转换sql的方法

using ExpressionDemo.DBExtend;
using System;
using System.Collections.Generic;
using System.Linq.Expressions;

namespace ExpressionDemo.Visitor
{
    public class ConditionBuilderVisitor : ExpressionVisitor
    {
        private Stack<string> _StringStack = new Stack<string>();

        public string Condition()
        {
            string condition = string.Concat(this._StringStack.ToArray());
            this._StringStack.Clear();
            return condition;
        }

        /// <summary>
        /// 如果是二元表达式
        /// </summary>
        /// <param name="node"></param>
        /// <returns></returns>
        protected override Expression VisitBinary(BinaryExpression node)
        {
            if (node == null) throw new ArgumentNullException("BinaryExpression");

            this._StringStack.Push(")");
            base.Visit(node.Right);//解析右边
            this._StringStack.Push(" " + node.NodeType.ToSqlOperator() + " ");
            base.Visit(node.Left);//解析左边
            this._StringStack.Push("(");

            return node;
        }
        /// <summary>
        /// 
        /// </summary>
        /// <param name="node"></param>
        /// <returns></returns>
        protected override Expression VisitMember(MemberExpression node)
        {
            if (node == null) throw new ArgumentNullException("MemberExpression");
            this._StringStack.Push(" [" + node.Member.Name + "] ");
            return node;
        }
        /// <summary>
        /// 常量表达式
        /// </summary>
        /// <param name="node"></param>
        /// <returns></returns>
        protected override Expression VisitConstant(ConstantExpression node)
        {
            if (node == null) throw new ArgumentNullException("ConstantExpression");
            this._StringStack.Push(" '" + node.Value + "' ");
            return node;
        }
        /// <summary>
        /// 方法表达式
        /// </summary>
        /// <param name="m"></param>
        /// <returns></returns>
        protected override Expression VisitMethodCall(MethodCallExpression m)
        {
            if (m == null) throw new ArgumentNullException("MethodCallExpression");

            string format;
            switch (m.Method.Name)
            {
                case "StartsWith":
                    format = "({0} LIKE {1}+'%')";
                    break;

                case "Contains":
                    format = "({0} LIKE '%'+{1}+'%')";
                    break;

                case "EndsWith":
                    format = "({0} LIKE '%'+{1})";
                    break;

                default:
                    throw new NotSupportedException(m.NodeType + " is not supported!");
            }
            this.Visit(m.Object);
            this.Visit(m.Arguments[0]);
            string right = this._StringStack.Pop();
            string left = this._StringStack.Pop();
            this._StringStack.Push(String.Format(format, left, right));

            return m;
        }
    }
}

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

namespace ExpressionDemo.DBExtend
{
    internal static class SqlOperator
    {
        internal static string ToSqlOperator(this ExpressionType type)
        {
            switch (type)
            {
                case (ExpressionType.AndAlso):
                case (ExpressionType.And):
                    return "AND";
                case (ExpressionType.OrElse):
                case (ExpressionType.Or):
                    return "OR";
                case (ExpressionType.Not):
                    return "NOT";
                case (ExpressionType.NotEqual):
                    return "<>";
                case ExpressionType.GreaterThan:
                    return ">";
                case ExpressionType.GreaterThanOrEqual:
                    return ">=";
                case ExpressionType.LessThan:
                    return "<";
                case ExpressionType.LessThanOrEqual:
                    return "<=";
                case (ExpressionType.Equal):
                    return "=";
                default:
                    throw new Exception("不支持该方法");
            }

        }
    }
}

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然后调用的时候如下：

{
    //修改表达式目录树
    Expression<Func<int, int, int>> exp = (m, n) => m * n + 2;
    OperationsVisitor visitor = new OperationsVisitor();               
    Expression expNew = visitor.Modify(exp);
}

{
    Expression<Func<People, bool>> lambda = x => x.Age > 5 && x.Id > 5
                                             && x.Name.StartsWith("1")
                                             && x.Name.EndsWith("1")
                                             && x.Name.Contains("1");

    string sql = string.Format("Delete From [{0}] WHERE {1}"
        , typeof(People).Name
        , " [Age]>5 AND [ID] >5"
        );
    ConditionBuilderVisitor vistor = new ConditionBuilderVisitor();
    vistor.Visit(lambda);
    Console.WriteLine(vistor.Condition());
}
{
    Expression<Func<People, bool>> lambda = x => x.Age > 5 && x.Name == "A" || x.Id > 5;
    ConditionBuilderVisitor vistor = new ConditionBuilderVisitor();
    vistor.Visit(lambda);
    Console.WriteLine(vistor.Condition());
}
{
    Expression<Func<People, bool>> lambda = x => x.Age > 5 || (x.Name == "A" && x.Id > 5);
    ConditionBuilderVisitor vistor = new ConditionBuilderVisitor();
    vistor.Visit(lambda);
    Console.WriteLine(vistor.Condition());
}
{
    Expression<Func<People, bool>> lambda = x => (x.Age > 5 || x.Name == "A") && x.Id > 5;
    ConditionBuilderVisitor vistor = new ConditionBuilderVisitor();
    vistor.Visit(lambda);
    Console.WriteLine(vistor.Condition());
}

View Code

目前Expression只支持ExpressionType的84种操作符Add， AndAlso等等，然后VisitMethodCall这个方法中表示lambda能解析出来的方法名字，如果需要可以自行修改会得到对应的sql语句的where条件！

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原文地址：https://www.cnblogs.com/bigbox777/p/12463156.html