目录
一、什么是表达式目录树
二、Func与Expression的区别
1、Func是方法
3、使用ILSpy反编译解析看一下
编辑
编辑
4、拼装练习
5、动态生成硬编码(通用、性能好)
5、表达式目录树动态生成的用途:
6、递归解析表达式目录树
7、表达式拼接
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些
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