- 技术博客
- (精华)2020年9月6日 C#基础知识点 Linq的使用场景
一：比较返回当前项目
namespace LinqSample
{
public class Quote {
public Stock Stock
{ get; set; }
public decimal Price
{ get; set; }
public DateTime Date
{ get; set; }
}
public class Stock {
public string Name
{ get; set; }
public List
<Quote
> Quotes
{ get; set; }
public override string ToString() {
return $
"{Name}: MIN {Quotes.Min(q => q.Price)} - MAX {Quotes.Max(q => q.Price)}";
}
}
public partial class Program
{
private static void Sample01()
{
var stock
= new Stock {Name
= "Stock Demo"};
stock
.Quotes
= new List<Quote
>
{
new Quote{Stock
= stock
, Price
= 200, Date
= DateTime
.Parse("2020/3/29")},
new Quote{Stock
= stock
, Price
= 150, Date
= DateTime
.Parse("2020/3/21")},
new Quote{Stock
= stock
, Price
= 220, Date
= DateTime
.Parse("2020/3/23")},
new Quote{Stock
= stock
, Price
= 180, Date
= DateTime
.Parse("2020/3/25")},
new Quote{Stock
= stock
, Price
= 100, Date
= DateTime
.Parse("2020/3/26")},
};
Console
.WriteLine( $
"Stock: {stock}");
var minQuote
= stock
.Quotes
.MinItem(q
=> q
.Date
);
Console
.WriteLine(minQuote
.Price
);
}
}
public static class Sample01Extensions
{
public static Quote MinPrice(this IEnumerable
<Quote
> source
)
{
return source
.Aggregate((t
, s
) => t
.Price
< s
.Price
? t
: s
);
}
public static TSouce
MinItem<TSouce, TCompareValue>(this IEnumerable
<TSouce
> source
,
Func
<TSouce
, TCompareValue
> comparerExpression
)
{
var comparer
= Comparer
<TCompareValue
>.Default
;
return source
.Aggregate((minValue
, item
) =>
{
var result
= comparer
.Compare(comparerExpression(minValue
), comparerExpression(item
));
return result
< 0 ? minValue
: item
;
});
}
}
}
二：改写扩展where，自定义默认条件返回
namespace LinqSample
{
public interface IVisible
{
bool Visible
{ get; set; }
}
public class Customer : IVisible
{
public string Name
{ get; set; }
public int Age
{ get; set; }
public bool Visible
{ get; set; }
public Customer()
{
Visible
= true;
}
public override string ToString()
{
return $
"{Name} 今年 {Age} 岁了";
}
}
public partial class Program
{
private static void Sample02()
{
var customers
= new List<Customer
>
{
new Customer{Name
= "张三", Age
= 20, Visible
= false},
new Customer{Name
= "李四", Age
= 18},
new Customer{Name
= "王五", Age
= 22},
};
var query
= from c
in customers
where c
.Age
< 22 select c
;
foreach (var customer
in query
)
{
Console
.WriteLine(customer
);
}
}
}
public static class Sample02Extensions
{
public static IEnumerable
<TSource
> Where<TSource>(
this IEnumerable
<TSource
> source
,
Func
<TSource
, bool> predicate
) where TSource
: IVisible
{
return Enumerable
.Where(source
, item
=> item
.Visible
== true && predicate(item
));
}
}
}
