Csharper中的表达式树
这节课来了解一下表示式树是什么？
在C#中，表达式树是一种数据结构，它可以表示一些代码块，如Lambda表达式或查询表达式。表达式树使你能够查看和操作数据，就像你可以查看和操作代码一样。它们通常用于创建动态查询和解析表达式。
一、认识表达式树
为什么要这样说？它和委托有什么区别？
创建一个简单的表达式树和委托
public class ExpressionDemo { void Show() { Func<int, bool> fun1 = x => x > 10; Expression<Func<int, bool>> expression1 = x => x > 10; } } 然后f12转到定义
public sealed class Expression<TDelegate> : LambdaExpression 尝试用大括号定义一个表达式树
debug运行后，用vs查看一下定义的表达式树对象.
发现表达式树一些特点:
- 可以通过lambda表达式来声明
- 是一个泛型类的接口，类型参数是一个委托
- Expression声明中，不能包含大括号.
- 通过VS展开查看，包含body(lamubda的主体部分)，描述了参数的名称和类型，描述了返回值的名称和类型； 展开body, body包含 左边是什么，右边是什么，式子的操作类型是什么.
结论:
表达式树,是一个计算式的描述，按照常规的计算逻辑，通过类的属性来进行描述多个节点之间的关系； 形似于一个树形结构----二叉树； 二叉树不断地去分解，可以得到这个式子中的任何一个独立的元素；----是一个二叉树，是一个数据结构； 如果需要可以把这个结构不断的拆解；得到中间的最小元素；在需要的时候，也可以通过每个元素，组装起来；
委托是一个类，而表达式树是一个二叉树的数据结构。
为了更加深入的了解表达式树，这里也使用ilspy进行反编译，以便于更加了解表达式树的本质.
这里使用一个比较复杂的表达式树的语句来方便我们去理解
Expression<Func<int ,int ,int>> expression2= (x, y) => x *y+2+3;
优化一下这段代码
//定义2个变量 ParameterExpression parameterExpression = Expression.Parameter(typeof(int), "x"); ParameterExpression parameterExpression2 = Expression.Parameter(typeof(int), "y"); //定义常量 var contact1 = Expression.Constant(2, typeof(int)); var contact2= Expression.Constant(3, typeof(int)); //定义表达式 x*y var MultiplyXy= Expression.Multiply(parameterExpression, parameterExpression2); //定义表达式 x*y的结果+2 var add1 = Expression.Add(MultiplyXy, contact1); //定义表达式 x*y+2的结果+3 var add2 = Expression.Add(add1, contact2); //定义最终的lambda表达式 Expression<Func<int, int, int>> expression2 = Expression.Lambda<Func<int, int, int>>(add2, new ParameterExpression[2] { parameterExpression, parameterExpression2 }); 如图所示的解析:
已经将相应的代码粘贴到上方，就是类似二叉树结构的因式分解，转换成为最小的子问题，最后解决一个需要解决的大问题。
二、动态拼装Expression
我们自己去拼装一个表达式树去理解表达式树的秘密.
首先创建一个People类
public class People { public int Age { get; set; } public string Name { get; set; } public int Id; } 下面来拼接一个比较复杂的表达式
Expression<Func<People, bool>> predicate = c => c.Id == 10 && c.Name.ToString().Equals("张三"); 对应的表达式树的代码
//定义一个People类型的参数 ParameterExpression parameterExpression = Expression.Parameter(typeof(People), "c"); //获取People的Id属性 PropertyInfo? propertyId = typeof(People).GetProperty("Id"); //定义10这个常量 ConstantExpression constantExpression = Expression.Constant(10, typeof(int)); //定义c.Id>10这个表达式 BinaryExpression left =Expression.GreaterThan(Expression.Property(parameterExpression, propertyId), constantExpression); //获取People的Name属性 PropertyInfo? propertyName = typeof(People).GetProperty("Name"); //c.Name MemberExpression memName = Expression.Property(parameterExpression, propertyName); //to string方法 MethodInfo? methodtostring=typeof(string).GetMethod("ToString",new Type[0]); //调用tostring方法 MethodCallExpression instance =Expression.Call(memName, methodtostring,Array.Empty<Expression>()); //获取equals方法 MethodInfo? methodEquals = typeof(string).GetMethod("Equals", new Type[] { typeof(string) }); //定义c.Name.ToString().Equals("张三")这个表达式 MethodCallExpression right = Expression.Call(instance, methodEquals, Expression.Constant("张三", typeof(string))); //定义c.Age<25这个表达式 PropertyInfo? propertyAge = typeof(People).GetProperty("Age"); ConstantExpression constantExpression2 = Expression.Constant(25, typeof(int)); BinaryExpression right2 = Expression.LessThan(Expression.Property(parameterExpression, propertyAge), constantExpression2); //定义c.Id>10 && c.Name.ToString().Equals("张三") && c.Age<25这个表达式 BinaryExpression and1 = Expression.AndAlso(left, right); BinaryExpression and2 = Expression.AndAlso(and1, right2); //定义最终的lambda表达式 Expression<Func<People, bool>> expression = Expression.Lambda<Func<People, bool>>(and2, new ParameterExpression[1] { parameterExpression }); //编译表达式 Func<People, bool> func = expression.Compile(); //调用表达式 People people = new People() { Id = 11, Name = "张三", Age = 20 }; Console.WriteLine(func(people)); 这样就拼接出来了需要的表达式树.
三、表达式树的应用价值
为什么要拼装这个表达式目录树呢?
现在主流的是Linq:
Linq to Sql -----把相同的逻辑封装，把不同的逻辑通过表达式目录树来传递；
传递表达式目录树：对应的是查询条件；在传递之前就应该把查询的条件拼装好；
例子
Expression<Func<People, bool>> expression2 = p => p.Id == 10 && p.Name.Equals("阳光下的微笑"); 拼接后的结果
//按关键字是否存在来拼装； Expression<Func<People, bool>> exp = p=> true; Console.WriteLine("用户输入个名称，为空就跳过"); string name = Console.ReadLine(); if (!string.IsNullOrWhiteSpace(name)) { //exp = p => p.Name.Contains(name); exp= exp.And(c=>c.Name.Contains(name)); } Console.WriteLine("用户输入个最小年纪，为空就跳过"); string age = Console.ReadLine(); if (!string.IsNullOrWhiteSpace(age) && int.TryParse(age, out int iAge)) { // exp = p => p.Age > iAge; exp = exp.And(p => p.Age > iAge); } 例子2
//Expression<Func<People, bool>> newExpress = x => x.Age > 5 && x.Id > 5 现在使用表达式树进行链接
Expression<Func<People, bool>> lambda1 = x => x.Age > 5; Expression<Func<People, bool>> lambda2 = x => x.Id > 5; //Expression<Func<People, bool>> newExpress = x => x.Age > 5 && x.Id > 5; Expression<Func<People, bool>> lambda3 = lambda1.And(lambda2); //且 两个都满足，通过&&链接 Expression<Func<People, bool>> lambda4 = lambda1.Or(lambda2);//或 两个只要有一个就可以 通过或者来链接 || Expression<Func<People, bool>> lambda5 = lambda1.Not();//非 这里实现了常见的且、或、非逻辑运算符的表达式
public static class ExpressionExtend { /// <summary> /// 合并表达式 expr1 AND expr2 /// </summary> /// <typeparam name="T"></typeparam> /// <param name="expr1"></param> /// <param name="expr2"></param> /// <returns></returns> public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2) { //return Expression.Lambda<Func<T, bool>>(Expression.AndAlso(expr1.Body, expr2.Body), expr1.Parameters); ParameterExpression newParameter = Expression.Parameter(typeof(T), "c"); NewExpressionVisitor visitor = new NewExpressionVisitor(newParameter); var left = visitor.Replace(expr1.Body); var right = visitor.Replace(expr2.Body); //为了能够生成一个新的表达式目录树 var body = Expression.And(left, right); return Expression.Lambda<Func<T, bool>>(body, newParameter); } /// <summary> /// 合并表达式 expr1 or expr2 /// </summary> /// <typeparam name="T"></typeparam> /// <param name="expr1"></param> /// <param name="expr2"></param> /// <returns></returns> public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1, Expression<Func<T, bool>> expr2) { ParameterExpression newParameter = Expression.Parameter(typeof(T), "c"); NewExpressionVisitor visitor = new NewExpressionVisitor(newParameter); var left = visitor.Replace(expr1.Body); var right = visitor.Replace(expr2.Body); var body = Expression.Or(left, right); return Expression.Lambda<Func<T, bool>>(body, newParameter); } public static Expression<Func<T, bool>> Not<T>(this Expression<Func<T, bool>> expr) { var candidateExpr = expr.Parameters[0]; var body = Expression.Not(expr.Body); return Expression.Lambda<Func<T, bool>>(body, candidateExpr); } } internal class NewExpressionVisitor : ExpressionVisitor { public ParameterExpression _NewParameter { get; private set; } public NewExpressionVisitor(ParameterExpression param) { this._NewParameter = param; } public Expression Replace(Expression exp) { return this.Visit(exp); } protected override Expression VisitParameter(ParameterExpression node) { return this._NewParameter; } } 现在有一个新的需求，需要把People拷贝到NewPeople这个新的类，来看下效率怎么样?
People和PeopleCopy类
public class People { public int Age { get; set; } public string Name { get; set; } public int Id; } /// <summary> /// 实体类Target /// PeopleDTO /// </summary> public class PeopleCopy { public int Age { get; set; } public string Name { get; set; } public int Id; } 直接赋值的方式
PeopleCopy peopleCopy1 = new PeopleCopy() { Id = people.Id, Name = people.Name, Age = people.Age }; 反射赋值的方式
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 propName = tIn.GetType().GetProperty(itemOut.Name); itemOut.SetValue(tOut, propName.GetValue(tIn)); } foreach (var itemOut in tOut.GetType().GetFields()) { var fieldName = tIn.GetType().GetField(itemOut.Name); itemOut.SetValue(tOut, fieldName.GetValue(tIn)); } return tOut; } } PeopleCopy peopleCopy2= ReflectionMapper.Trans<People, PeopleCopy>(people); json序列化的方式
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) { string strTin = JsonConvert.SerializeObject(tIn); return JsonConvert.DeserializeObject<TOut>(strTin); } } PeopleCopy peopleCopy3 = SerializeMapper.Trans<People, PeopleCopy>(people); 表达式目录树的方式
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)) { #region 这里是拼装---赋属性值的代码 ParameterExpression parameterExpression = Expression.Parameter(typeof(TIn), "p"); //MemberBinding: 就是一个表达式目录树 List<MemberBinding> memberBindingList = new List<MemberBinding>(); foreach (var item in typeof(TOut).GetProperties()) //这里是处理属性的 { 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()) //处理字段的 { 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()); //组装了一个转换的过程； Expression<Func<TIn, TOut>> lambda = Expression.Lambda<Func<TIn, TOut>>(memberInitExpression, new ParameterExpression[] { parameterExpression }); #endregion Func<TIn, TOut> func = lambda.Compile();//拼装是一次性的 _Dic[key] = func; } return ((Func<TIn, TOut>)_Dic[key]).Invoke(tIn); } } PeopleCopy peopleCopy4 = ExpressionMapper.Trans<People, PeopleCopy>(people); 表达式+反射+泛型类的方式
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)); 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 = lambda.Compile();//拼装是一次性的 } public static TOut Trans(TIn t) { return _FUNC(t); } } } PeopleCopy peopleCopy5 = ExpressionGenericMapper<People, PeopleCopy>.Trans(people); 最后运行一百万次，来看一下效率。
{ People people = new People() { Id = 11, Name = "Richard", Age = 31 }; long common = 0; long generic = 0; long cache = 0; long reflection = 0; long serialize = 0; { Stopwatch watch = new Stopwatch(); watch.Start(); for (int i = 0; i < 1_000_000; 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 < 1_000_000; i++) { PeopleCopy peopleCopy = ReflectionMapper.Trans<People, PeopleCopy>(people); } watch.Stop(); reflection = watch.ElapsedMilliseconds; } { Stopwatch watch = new Stopwatch(); watch.Start(); for (int i = 0; i < 1_000_000; i++) { PeopleCopy peopleCopy = SerializeMapper.Trans<People, PeopleCopy>(people); } watch.Stop(); serialize = watch.ElapsedMilliseconds; } { Stopwatch watch = new Stopwatch(); watch.Start(); for (int i = 0; i < 1_000_000; i++) { PeopleCopy peopleCopy = ExpressionMapper.Trans<People, PeopleCopy>(people); } watch.Stop(); cache = watch.ElapsedMilliseconds; } { Stopwatch watch = new Stopwatch(); watch.Start(); for (int i = 0; i < 1_000_000; i++) { PeopleCopy peopleCopy = ExpressionGenericMapper<People, PeopleCopy>.Trans(people); } watch.Stop(); generic = watch.ElapsedMilliseconds; } Console.WriteLine($"common = {common} ms"); //性能最高，但是不能通用； Console.WriteLine($"reflection = {reflection} ms"); Console.WriteLine($"serialize = {serialize} ms"); Console.WriteLine($"cache = {cache} ms"); Console.WriteLine($"generic = {generic} ms"); //性能好，而且扩展性也好===又要马儿跑，又要马儿不吃草。。。 } 看运行后的结果
核心：动态生成硬编码；----代码运行的时候生成了一段新的逻辑；
四、表达式树和sql
为什么要使用表达式目录树来拼装解析呢？
可以提供重用性
如果封装好一个方法，接受一个表达式树，在解析的时候，其实就是不断的访问，访问的时候，会按照固定的规则，避免出错;
任何的一个表达式树都可以用一个通用的方法解析并且支持泛型,更加容易去封装;
例子:
需要的扩展类
public class OperationsVisitor : ExpressionVisitor { public Expression Modify(Expression expression) { Console.WriteLine(expression.ToString()) ; //ExpressionVisitor: //1.Visit方法--访问表达式目录树的入口---分辨是什么类型的表达式目录 //2.调度到更加专业的方法中进一步访问,访问一遍之后，生成一个新的表达式目录 ---有点像递归，不全是递归； //3.因为表达式目录树是个二叉树，ExpressionVisitor一直往下访问，一直到叶节点；那就访问了所有的节点 //4.在访问的任何一个环节，都可以拿到对应当前环节的内容(参数名称、参数值。。)，就可以进一步扩展 return this.Visit(expression); } /// <summary> /// 覆写父类方法 /// </summary> /// <param name="b"></param> /// <returns></returns> 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); } else if (b.NodeType==ExpressionType.Multiply) //如果是相乘 { Expression left = this.Visit(b.Left); Expression right = this.Visit(b.Right); return Expression.Divide(left, right); //相除 } return base.VisitBinary(b); } /// <summary> /// 覆写父类方法 /// </summary> /// <param name="node"></param> /// <returns></returns> protected override Expression VisitConstant(ConstantExpression node) { return base.VisitConstant(node); } 对应的表达式解析
Expression<Func<int, int, int>> exp = (m, n) => m * n + 2; OperationsVisitor visitor = new OperationsVisitor(); //visitor.Visit(exp); Expression expNew = visitor.Modify(exp); 同时表达式树中已经通过使用观察者模式封装好了Visit方法.
- Visit方法--访问表达式树的入口---分辨是什么类型的表达式目录
- 调度到更加专业的方法中进一步访问，访问一边以后，生成一个新的表达式目录. --- 有点像递归，不全是递归
- 因为表达式目录树是一个二叉树，ExpreesionVistor一直往下访问，一直到叶子节点;通过二叉树的遍历就访问了所有的节点.
- 在访问的任何一个环节，都可以拿到对应当前环节的内容(参数名称、参数值...)就可以进一步扩展.
现在开始将表达式树跟sql语句进行连接
例子:
扩展类
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> /// 解析属性 /// </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; if (node.Expression is ConstantExpression) { var value1 = this.InvokeValue(node); var value2 = this.ReflectionValue(node); //this.ConditionStack.Push($"'{value1}'"); this._StringStack.Push("'" + value2 + "'"); } else { this._StringStack.Push(" [" + node.Member.Name + "] "); } return node; } private object InvokeValue(MemberExpression member) { var objExp = Expression.Convert(member, typeof(object));//struct需要 return Expression.Lambda<Func<object>>(objExp).Compile().Invoke(); } private object ReflectionValue(MemberExpression member) { var obj = (member.Expression as ConstantExpression).Value; return (member.Member as FieldInfo).GetValue(obj); } /// <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; } } 对应的sql语句的解析
{ Expression<Func<People, bool>> lambda = x => x.Age > 5 && x.Id > 5 && x.Name.StartsWith("1") // like '1%' && x.Name.EndsWith("1") // like '%1' && x.Name.Contains("1");// like '%1%' string sql = string.Format("Delete From [{0}] WHERE [Age]>5 AND [ID] >5" , typeof(People).Name , " [Age]>5 AND [ID] >5"); ConditionBuilderVisitor vistor = new ConditionBuilderVisitor(); vistor.Visit(lambda); Console.WriteLine(vistor.Condition()); } { // ((( [Age] > '5') AND( [Name] = [name] )) OR( [Id] > '5' )) string name = "AAA"; Expression<Func<People, bool>> lambda = x => x.Age > 5 && x.Name == name || 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()); } 在我自己的看法，使用表达式树而不是传统的方式去解析sql语句的优点
- 通过二叉树的方式表达，更加的有条理性
- 使用泛型等技术更方式实现一个通用的sql语句的解析。
- 会有类型检查，出错后也能使用异常处理。
