一、什么是表達(dá)式目錄樹(shù)

（1）Expression我們稱(chēng)為是表達(dá)式樹(shù)，是一種數(shù)據(jù)結(jié)構(gòu)體，用于存儲(chǔ)需要計(jì)算，運(yùn)算的一種結(jié)構(gòu)，這種結(jié)構(gòu)可以只是存儲(chǔ)，而不進(jìn)行運(yùn)算。通常表達(dá)式目錄樹(shù)是配合Lambda一起來(lái)使用的，lambda可以是匿名方法，當(dāng)然也可以使用Expression來(lái)動(dòng)態(tài)的創(chuàng)建！

二、Func與Expression的區(qū)別

1、Func是方法

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

?2、Expression是數(shù)據(jù)結(jié)構(gòu)

//lambda表達(dá)式聲明表達(dá)式目錄樹(shù)
Expression<Func<int, int, int>> exp = (m, n) => m * n + 2;
int result = exp.Compile().Invoke(1, 2);
Console.WriteLine(result); 
//運(yùn)算：1*2+2=4

注意：Expression只能為1行（如下圖會(huì)報(bào)錯(cuò)）

3、使用ILSpy反編譯解析看一下

調(diào)一下格式更好看一點(diǎn)

            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});

打印看看結(jié)果

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

得到134，與m*n+2得出結(jié)果一致

4、拼裝練習(xí)

（1）練習(xí)一：

（2）練習(xí)二

（3）練習(xí)三

5、動(dòng)態(tài)生成硬編碼（通用、性能好）

需求：我希望復(fù)制一個(gè)People出來(lái)

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

方法1：通過(guò)硬編碼直接賦值

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

方法2：通過(guò)反射賦予

方法3：通過(guò)Json序列化與反序列化賦值

第一種方法性能最好，但是不夠通用。方法2和方法3性能不好。

方法4：

這時(shí)候可以考慮使用表達(dá)式目錄樹(shù)來(lái)動(dòng)態(tài)生成硬編碼

思路：用表達(dá)目錄樹(shù)動(dòng)態(tài)生成硬編碼，生成保存到字典里，下次再調(diào)用的時(shí)候則直接從字典里拿。

 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：泛型緩存（相比方法4可以節(jié)省讀取字典時(shí)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);}

看一下字典緩存和泛型類(lèi)緩存消耗的時(shí)間，明顯可以看到通過(guò)泛型類(lèi)緩存的性能更好，因?yàn)楣?jié)省了查找字典的性能消耗。

         PrintExecutionTime(() =>{Console.WriteLine("通過(guò)字典緩存，第一次消耗時(shí)間：");PeopleCopy copy = ExpressionMapper.Trans<People, PeopleCopy>(new People() { Age = 10, Name = "哇哈哈" });});PrintExecutionTime(() =>{Console.WriteLine("通過(guò)字典緩存，第二次消耗時(shí)間：");PeopleCopy copy2 = ExpressionMapper.Trans<People, PeopleCopy>(new People() { Age = 10, Name = "哇哈哈" });});PrintExecutionTime(() =>{Console.WriteLine("通過(guò)泛型類(lèi)緩存，第一次消耗時(shí)間：");PeopleCopy copy3 = ExpressionGenericMapper<People, PeopleCopy>.Trans(new People() { Age = 11, Name = "啦啦啦" });});PrintExecutionTime(() =>{Console.WriteLine("通過(guò)泛型類(lèi)緩存，第二次消耗時(shí)間：");PeopleCopy copy4 = ExpressionGenericMapper<People, PeopleCopy>.Trans(new People() { Age = 11, Name = "啦啦啦" });});

5、表達(dá)式目錄樹(shù)動(dòng)態(tài)生成的用途：

可以用來(lái)替代反射，因?yàn)榉瓷淇梢酝ㄓ?#xff0c;但是性能不夠

可以生成硬編碼，可以提升性能

6、遞歸解析表達(dá)式目錄樹(shù)

（1）ExpressionVisitor：肯定得遞歸解析表達(dá)式目錄樹(shù)，因?yàn)椴恢郎疃鹊囊豢脴?shù)

（2）只有一個(gè)入口叫Visit

（3）首先檢查是個(gè)什么類(lèi)型的表達(dá)式，然后調(diào)用對(duì)應(yīng)的protected virtual

（4）得到結(jié)果繼續(xù)去檢查類(lèi)型——調(diào)用對(duì)應(yīng)的Visit方法——再檢測(cè)——再調(diào)用。。。

案例：解析（m*n+2）

第一步（把m*n+2傳入，入口時(shí)Visit）

第二步：檢測(cè)到二元表達(dá)式，m*n+2進(jìn)入VisitBinary方法.

node.Left為m*n（這里會(huì)再次調(diào)用VisitBinary方法）? ? node.Right為2

??第三步：m*n也時(shí)二元表達(dá)式，因此重新進(jìn)入VisitBinary方法

node.Left為m（進(jìn)入VisitParameter方法）? node.Right為n（進(jìn)入VisitParameter方法）

第四步：m*n解析完開(kāi)始解析2，會(huì)進(jìn)入VisitConstant方法

基礎(chǔ)應(yīng)用：我們可以把表達(dá)式的所有+號(hào)變成-號(hào)

從下圖可以發(fā)現(xiàn)，表達(dá)式expression變成了m*n-2

7、表達(dá)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();}

調(diào)試成功，ID>5且age>5的只有這兩個(gè)人。

表達(dá)式樹(shù)也可以通過(guò)這種辦法進(jìn)行查詢(xún)操作