链式-父类中返回子类对象

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链式-父类中返回子类对象

一晃五年没写博客了，依旧再C#上耕耘，依旧没有啥建树，现在也不知道.net上还有多少人再使用，在这里分享一些自己觉得写的还算优雅的代码。

对于自己写着完的代码，我特别喜欢链式（来源于jQuery的影响吧），大部分时候链式就是将返回值为void类型的对象，返回this指针，直到我遇到一个特殊情况——在父类中返回子类类型。大部分情况父类都不知道子类有什么，根本没有返回子类的需求

但是由于链式调用父类的接口返回父类对象，就无法继续链式了。说明可能不清楚，直接show code

 1  public class OldWhereV2<T>
 2     {
 3         protected Expression<Func<T, bool>> expression = null;
 4 
 5         public OldWhereV2<T> Where(Expression<Func<T, bool>> memberExpression)
 6         {
 7             return this;
 8         }
 9 
10         public OldWhereV2<T> Or(Expression<Func<T, bool>> memberExpression)
11         {
12             return this;
13         }
14 
15         public OldWhereV2<T> Add(Expression<Func<T, bool>> memberExpression)
16         {
17             return this;
18         }
19     }
20 
21     public class OldQeuryV2<T> : OldWhereV2<T>
22     {
23 
24         public OldQeuryV2<T> Select(Expression<Func<T, object>> memberExpression)
25         {
26             return this;
27         }
28 
29         public OldQeuryV2<T> Take(int count)
30         {
31             return this;
32         }
33 
34         public OldQeuryV2<T> Order(Expression<Func<T, object>> memberExpression, bool asc)
35         {
36             return this;
37         }
38     }

调用的时候，如果使用链式

1  var query =new OldQeuryV2<Train>()
2                .Select(x => x.Apply_Time)
3                .Select(x => x.Apply_Time)
4                .Select(x => x.Approval_OrgName)
5                .Where(x => x.Create_Time > DateTime.Now)
6                .Add(x => x.Approval_OrgName == "")
7                .Order(x => x.Approval_OrgGID, true)
8                .Order(x => x.Apply_Time, false)
9                .Take(10);

.Order(x => x.Approval_OrgGID, true) 这行代码会报错的。因为Where返回的是OldWhereV2<T>类型，而Order方法要求OldQeuryV2<T>类型

这个问题困扰我一晚，后来我记得在哪里看过一本书，书中有泛型自包含的例子，但是当时完全看不懂，但是此处感觉使用完全没毛病所以就做了简单修改

 1  public abstract class Condition<T, M> where M : Condition<T, M>
 2     {
 3         protected Expression<Func<T, bool>> expression = null;
 4 
 5         public M Where(Expression<Func<T, bool>> memberExpression)
 6         {
 7             expression = memberExpression;
 8             return (M)this;
 9         }
10 
11         public M Or(Expression<Func<T, bool>> memberExpression)
12         {
13             if (expression == null)
14             {
15                 expression = memberExpression;
16             }
17             else
18             {
19                 var invokedExpr = Expression.Invoke(memberExpression, expression.Parameters.Cast<Expression>());
20                 expression = Expression.Lambda<Func<T, bool>>(Expression.OrElse(expression.Body, invokedExpr), expression.Parameters);
21             }
22             return (M)this;
23         }
24 
25         public M Add(Expression<Func<T, bool>> memberExpression)
26         {
27             if (expression == null)
28             {
29                 expression = memberExpression;
30             }
31             else
32             {
33                 var invokedExpr = Expression.Invoke(memberExpression, expression.Parameters.Cast<Expression>());
34                 expression = Expression.Lambda<Func<T, bool>>(Expression.AndAlso(expression.Body, invokedExpr), expression.Parameters);
35             }
36             return (M)this;
37         }
38     }
39 
40     public class Qeury<T> : Condition<T, Qeury<T>>
41     {
42         List<MemberInfo> selects = new List<MemberInfo>();
43         Dictionary<MemberInfo, bool> orders = new Dictionary<MemberInfo, bool>();
44         int count = 1000;
45 
46         public Qeury<T> Select(Expression<Func<T, object>> memberExpression)
47         {
48             MemberInfo memberInfo = memberExpression.GetMemberInfo();
49             if (!selects.Contains(memberInfo))
50             {
51                 selects.Add(memberInfo);
52             }
53             return this;
54         }
55 
56         public Qeury<T> Take(int count)
57         {
58             this.count = count;
59             return this;
60         }
61 
62         public Qeury<T> Order(Expression<Func<T, object>> memberExpression, bool asc)
63         {
64             MemberInfo memberInfo = memberExpression.GetMemberInfo();
65             if (orders.ContainsKey(memberInfo))
66             {
67                 orders[memberInfo] = asc;
68             }
69             else
70             {
71                 orders.Add(memberInfo, asc);
72             }
73             return this;
74         }
75 
76         public string QeurySql()
77         {
78             var queryInfo = new QueryInfo()
79             {
80                 WhereExpression = this.expression,
81                 SelectFields = this.selects,
82                 Orders = this.orders,
83                 Count = this.count
84             };
85 
86             return TableAnalysis.GetTableInfo(typeof(T)).QeurySql(queryInfo);
87         }
88     }

这里将Condition<T>类修改为Condition<T,M> 而M是Condition<T,M>的子类，返回的时候只需要返回M类型就好了，当然由于Condition返回了子类，所以我把它设置成了抽象类，但是也可以不用。由于Qeury<T> :实现了Condition<T, Qeury<T>>，所以子类就可以正常调用父类的方法了。

具体例子如下：

1 var query =new Qeury<Train>()
2                .Select(x => x.Apply_Time)
3                .Select(x => x.Apply_Time)
4                .Select(x => x.Approval_OrgName)
5                .Where(x => x.Create_Time > DateTime.Now)
6                .Add(x => x.Approval_OrgName == "")
7                .Order(x => x.Approval_OrgGID, true)
8                .Order(x => x.Apply_Time, false)
9                .Take(10);

这个算是奇技淫巧，发出来给大家看看，不过不链式不久没有烦恼了吗，正常如下面定义就好了

			
   public class OldCondition<T>
   {
       public void Where(Expression<Func<T, bool>> memberExpression)
       {

       }

       public void Or(Expression<Func<T, bool>> memberExpression)
       {

       }

       public void Add(Expression<Func<T, bool>> memberExpression)
       {

       }
   }

   public class OldQeury<T> : OldCondition<T>
   {
       public void Select(Expression<Func<T, object>> memberExpression)
       {

       }

       public void Take(int count)
       {

       }

       public void Order(Expression<Func<T, object>> memberExpression, bool asc)
       {

       }
   }
			

出处：https://www.cnblogs.com/watermoon2/p/17412967.html

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