C# .Net学习笔记—— Expression 表达式目录树

一、什么是表达式目录树

（1）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！

二、Func与Expression的区别

1、Func是方法

 Func<int, int, int> func = (m, n) => m * n + 2;Console.WriteLine(func.Invoke(1, 1)); 
//运算：1*1+2=3

 2、Expression是数据结构

//lambda表达式声明表达式目录树
Expression<Func<int, int, int>> exp = (m, n) => m * n + 2;
int result = exp.Compile().Invoke(1, 2);
Console.WriteLine(result); 
//运算：1*2+2=4

注意：Expression只能为1行（如下图会报错）

3、使用ILSpy反编译解析看一下

调一下格式更好看一点

            ParameterExpression parameterExpression = Expression.Parameter(typeof(int), "m");ParameterExpression parameterExpression2 = Expression.Parameter(typeof(int), "n");var multiply = Expression.Multiply(parameterExpression, parameterExpression2);var constant = Expression.Constant(2,typeof(int));var add = Expression.Add(multiply, constant);Expression<Func<int, int, int>> exp =Expression.Lambda<Func<int, int, int>>(add,new ParameterExpression[2]{parameterExpression, parameterExpression2});

打印看看结果

      int result = exp.Compile().Invoke(11, 12);Console.WriteLine(result);

得到134，与m*n+2得出结果一致

4、拼装练习

（1）练习一：

（2）练习二

（3）练习三

5、动态生成硬编码（通用、性能好）

需求：我希望复制一个People出来

    public class People {public int Age;public string Name;}public class PeopleCopy {public int Age;public string Name;}

方法1：通过硬编码直接赋值

People people = new People()
{Age = 18,Name = "吴彦祖"
};
PeopleCopy peopleCopy = new PeopleCopy()
{Age = people.Age,Name = people.Name,
};

方法2：通过反射赋予

方法3：通过Json序列化与反序列化赋值

第一种方法性能最好，但是不够通用。方法2和方法3性能不好。

方法4：

这时候可以考虑使用表达式目录树来动态生成硬编码

思路：用表达目录树动态生成硬编码，生成保存到字典里，下次再调用的时候则直接从字典里拿。

 public class ExpressionMapper{private static Dictionary<string, object> _dic = new Dictionary<string, object>();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 field = Expression.Field(parameterExpression, typeof(TIn).GetField(item.Name));MemberBinding memberBinding = Expression.Bind(item, field);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});_dic[key] = lambda.Compile();}return ((Func<TIn, TOut>)_dic[key]).Invoke(tIn);}

方法5：泛型缓存（相比方法4可以节省读取字典时候的消耗）

 public class ExpressionGenericMapper<TIn, TOut>{private static Func<TIn, TOut> _Func;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 field = Expression.Field(parameterExpression, typeof(TIn).GetField(item.Name));MemberBinding memberBinding = Expression.Bind(item, field);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();}public static TOut Trans(TIn t) {return _Func(t);}

看一下字典缓存和泛型类缓存消耗的时间，明显可以看到通过泛型类缓存的性能更好，因为节省了查找字典的性能消耗。

         PrintExecutionTime(() =>{Console.WriteLine("通过字典缓存，第一次消耗时间：");PeopleCopy copy = ExpressionMapper.Trans<People, PeopleCopy>(new People() { Age = 10, Name = "哇哈哈" });});PrintExecutionTime(() =>{Console.WriteLine("通过字典缓存，第二次消耗时间：");PeopleCopy copy2 = ExpressionMapper.Trans<People, PeopleCopy>(new People() { Age = 10, Name = "哇哈哈" });});PrintExecutionTime(() =>{Console.WriteLine("通过泛型类缓存，第一次消耗时间：");PeopleCopy copy3 = ExpressionGenericMapper<People, PeopleCopy>.Trans(new People() { Age = 11, Name = "啦啦啦" });});PrintExecutionTime(() =>{Console.WriteLine("通过泛型类缓存，第二次消耗时间：");PeopleCopy copy4 = ExpressionGenericMapper<People, PeopleCopy>.Trans(new People() { Age = 11, Name = "啦啦啦" });});

5、表达式目录树动态生成的用途：

可以用来替代反射，因为反射可以通用，但是性能不够

可以生成硬编码，可以提升性能

6、递归解析表达式目录树

（1）ExpressionVisitor：肯定得递归解析表达式目录树，因为不知道深度的一棵树

（2）只有一个入口叫Visit

（3）首先检查是个什么类型的表达式，然后调用对应的protected virtual

（4）得到结果继续去检查类型——调用对应的Visit方法——再检测——再调用。。。

案例：解析（m*n+2）

第一步（把m*n+2传入，入口时Visit）

第二步：检测到二元表达式，m*n+2进入VisitBinary方法.

node.Left为m*n（这里会再次调用VisitBinary方法）    node.Right为2

  第三步：m*n也时二元表达式，因此重新进入VisitBinary方法

node.Left为m（进入VisitParameter方法）  node.Right为n（进入VisitParameter方法）

第四步：m*n解析完开始解析2，会进入VisitConstant方法

基础应用：我们可以把表达式的所有+号变成-号

从下图可以发现，表达式expression变成了m*n-2

7、表达式拼接

using System.Linq.Expressions;namespace ConsoleApp1
{public static class ExpressionExtend{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));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.And(left, right);return Expression.Lambda<Func<T, bool>>(body, newParameter);}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, int>> expr) {var candidateExpr = expr.Parameters[0];var body = Expression.Not(expr.Body);return Expression.Lambda<Func<T, bool>>(body, candidateExpr);}}public 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;}}
}

       public static void Show(){Expression<Func<People, bool>> lambdal = x => x.Age > 5;Expression<Func<People, bool>> lambda2 = x => x.ID > 5;Expression<Func<People, bool>> lambda3 = lambdal.And(lambda2);Expression<Func<People, bool>> lambda4 = lambdal.Or(lambda2);Expression<Func<People, bool>> lambda5 = lambdal.Not();var peopleList = Do(lambda3);for (int i = 0; i < peopleList.Count; i++){Console.WriteLine(peopleList[i].Name);}}private static List<People> Do(Expression<Func<People, bool>> func){List<People> people = new List<People>(){new People() { ID = 1, Age = 10, Name = "老一" },new People() { ID = 2, Age = 20, Name = "老二" },new People() { ID = 3, Age = 60, Name = "老三" },new People() { ID = 4, Age = 18, Name = "老四" },new People() { ID = 5, Age = 10, Name = "哈哈哈" },new People() { ID = 6, Age = 15, Name = "方式" },new People() { ID = 7, Age = 10, Name = "老六啦啦啦啦啦啦啦" },};return people.Where(func.Compile()).ToList();}

调试成功，ID>5且age>5的只有这两个人。

表达式树也可以通过这种办法进行查询操作