C#提供运算符重载功能,但这功能使用的场合并不多,相信很多C#开发人员虽然了解到有这一功能,但相信用到的比较少.为什么要自己重载运算符来生成SQL而不去用Linq?其目的也是非常简单的使用简单和灵活。先来看一下有多少运算符可以重载:+, -, *, /, %, &, |, ^, <<, >>==, !=, <, >, <=, >=
看上去还是挺多的,应该能满SQL对应的需要,首先整理出一个对应关系
c# SQL
== =
!= <>
> >
>= >=
< <
<= <=
& and
| or
总得来说基础的已经差不多了,但决少like,in等,这些可以使用函数或一技巧上的转换实现.
既然实现运算符重载,那当然就要重建一个对象做他基础实现,其作用就是类似于SQL中的字段.
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
|
public class FieldInfo { public FieldInfo(string table, string name) { DBContext.Init(); mTable = table; mName = name; } private string mTable; public string Table { get { return mTable; } } private string mName; public string Name { get { return mName; } }} |
表达一个字段的类型以上描述就足够了,有所在的表名和字段名.在实现运算符重载前还是把基础功能用函数实现,运算符重载方法简单调用就OK了.
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
|
public Expression Eq(object value) { string p = Expression.GetParamName(); Expression exp = new Expression(); exp.SqlText.Append(string.Format(" {0}=@{1} ",Name,p)); exp.Parameters.Add(new Command.Parameter{ Name=p, Value=Mappings.PropertyCastAttribute.CastValue(Table,Name, value)}); return exp; } public Expression LtEq(object value) { string p = Expression.GetParamName(); Expression exp = new Expression(); exp.SqlText.Append(string.Format(" {0}<=@{1} ", Name, p)); exp.Parameters.Add(new Command.Parameter { Name = p, Value = Mappings.PropertyCastAttribute.CastValue(Table, Name, value) }); return exp; } public Expression Lt(object value) { string p = Expression.GetParamName(); Expression exp = new Expression(); exp.SqlText.Append(string.Format(" {0}<@{1} ", Name, p)); exp.Parameters.Add(new Command.Parameter { Name = p, Value = Mappings.PropertyCastAttribute.CastValue(Table, Name, value) }); return exp; } |
大体上描述几个方法实现就行了,对于其他实现原理一样.下面开始运算重载部分
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
|
public static Expression operator ==(FieldInfo field, object value) { if (value == null) return field.IsNull(); if (value is System.Collections.IEnumerable && value.GetType() !=typeof(string)) return field.In((System.Collections.IEnumerable)value); return field.Eq(value); } public static Expression operator !=(FieldInfo field, object value) { if (value == null) return field.IsNotNull(); if (value is System.Collections.IEnumerable && value.GetType() != typeof(string)) return field.NotIn((System.Collections.IEnumerable)value); return field.NotEq(value); } public static Expression operator >(FieldInfo field, object value) { return field.Gt(value); } public static Expression operator >=(FieldInfo field, object value) { return field.GtEq(value); } public static Expression operator <(FieldInfo field, object value) { return field.Lt(value); } public static Expression operator <=(FieldInfo field, object value) { return field.LtEq(value); } |
到这里工作算是完成了,不过好象少了点什么东西似的...似乎没有实现&和|;因为这两个运算不是比较运算符所以不是FieldInfo对象实现的.以上代码中每个比较运算都返回了一个Expression对象,那&和|自然就由它来实现了
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
|
public static Expression operator &(Expression exp1, Expression exp2) { if (exp1 == null || exp1.SqlText.Length == 0) return exp2; if (exp2 == null || exp2.SqlText.Length == 0) return exp1; Expression exp = new Expression(); exp.SqlText.Append("("); exp.SqlText.Append(exp1.ToString()); exp.SqlText.Append(")"); exp.Parameters.AddRange(exp1.Parameters); exp.SqlText.Append(" and ("); exp.SqlText.Append(exp2.SqlText.ToString()); exp.SqlText.Append(")"); exp.Parameters.AddRange(exp2.Parameters); return exp; } public static Expression operator |(Expression exp1, Expression exp2) { if (exp1 == null || exp1.SqlText.Length == 0) return exp2; if (exp2 == null || exp2.SqlText.Length == 0) return exp1; Expression exp = new Expression(); exp.SqlText.Append("("); exp.SqlText.Append(exp1.ToString()); exp.SqlText.Append(")"); exp.Parameters.AddRange(exp1.Parameters); exp.SqlText.Append(" or ("); exp.SqlText.Append(exp2.SqlText.ToString()); exp.SqlText.Append(")"); exp.Parameters.AddRange(exp2.Parameters); return exp; } |
对于以上完整代码可以从https://smarkdata.svn.codeplex.com/svn/Smark/Smark.Data/Smark.Data/Expression.cs获取
实现代码的都完成的,那看一下分别在不同查询的情况下是什么效果:
sql:
select * from customer where region='UK'
c#
(Customer.Region=='UK').List<Customer>()
sql:
select * from Orders where OrderDate>'1998-7-8' and OrderDate <' 1998-8-8'
c#
(Order.OrderDate >"1998-7-8" & Order.OrderDate<"1998-8-8").List<Orders>()
sql:
select * from Orders where CustomerID in('2','5','6')
c#
(Orders.CustomerID ==new []{"2","5","6"}).List<Orders>();
sql:
select * from Orders where CustomerID in(select customerid from customer where region='UK')
c#
(Orders.CustomerID==Customer.CustomerID[Customer.Region=="UK"]).List<Orders>();
从以上代码可以看出由于自己重载所以自由度很高,对于==这个运算符可以代替很多SQL的比较操作如:=,in,in(select)等,当然还可以发挥编写者了想象力.
由于是自定义编写实现,那条件动态组合当然要比Linq所灵活很多:
|
1
2
3
4
5
|
Expression exp;if(a) exp &=order.id=="a";if(b) exp &= order.customerid==customer.customerid[customer.region=="b"] |