c#扩展方法奇思妙用—基础篇 八:Distinct 扩展
试想如果能写成下面的样子,是不是更简单优雅:
var p1 = products.Distinct(p => p.ID);
var p2 = products.Distinct(p => p.Name);
使用一个简单的lambda 作为参数,也符合Linq 一贯的风格。
可通过扩展方法实现:
Distinct 扩展方法
首先,创建一个通用比较的类,实现IEqualityComparer<T> 接口:
using System;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
using System.Linq;
public class CommonEqualityComparer<T, V> : IEqualityComparer<T>
{
private Func<T, V> keySelector;
public CommonEqualityComparer(Func<T, V> keySelector)
{
this.keySelector = keySelector;
}
public bool Equals(T x, T y)
{
return EqualityComparer<V>.Default.Equals(keySelector(x), keySelector(y));
}
public int GetHashCode(T obj)
{
return EqualityComparer<V>.Default.GetHashCode(keySelector(obj));
}
}
第17 行,用到了EqualityComparer<T> 类,本文最后有简要说明。
借助上面这个类,Distinct 扩展方法就很好写了:
1
public static class DistinctExtensions
{
public static IEnumerable<T> Distinct<T, V>(this IEnumerable<T> source, Func<T, V> keySelector)
{
return source.Distinct(new CommonEqualityComparer<T, V>(keySelector));
}
}
呵呵,简单吧!
Distinct 使用示例
根据ID :
var data1 = new Person[] {
new Person{ ID = 1, Name = "鹤冲天"},
new Person{ ID = 1, Name = "ldp"}
};
var ps1 = data1
.Distinct(p => p.ID)
.ToArray();
根据Name:
var data2 = new Person[] {
new Person{ ID = 1, Name = "鹤冲天"},
new Person{ ID = 2, Name = "鹤冲天"}
};
var ps2 = data2
.Distinct(p => p.Name)
.ToArray();
看了回复后,我做了些改进,推荐使用下面的方式:
改进
回复中有朋友提到“不区分大小写地排除重复的字符串”,也不难实现,只需要把上面的代码改进下就OK:
CommonEqualityComparer<T, V> 类:
using System;
using System.Collections.Generic;
using System.Runtime.CompilerServices;
using System.Linq;
public class CommonEqualityComparer<T, V> : IEqualityComparer<T>
{
private Func<T, V> keySelector;
private IEqualityComparer<V> comparer;
public CommonEqualityComparer(Func<T, V> keySelector, IEqualityComparer<V> comparer)
{
this.keySelector = keySelector;
this.comparer = comparer;
}
public CommonEqualityComparer(Func<T, V> keySelector)
: this(keySelector, EqualityComparer<V>.Default)
{ }
public bool Equals(T x, T y)
{
return comparer.Equals(keySelector(x), keySelector(y));
}
public int GetHashCode(T obj)
{
return comparer.GetHashCode(keySelector(obj));
}
}
Distinct 扩展方法:
public static class DistinctExtensions
{
public static IEnumerable<T> Distinct<T, V>(this IEnumerable<T> source, Func<T, V> keySelector)
{
return source.Distinct(new CommonEqualityComparer<T, V>(keySelector));
}
public static IEnumerable<T> Distinct<T, V>(this IEnumerable<T> source, Func<T, V> keySelector, IEqualityComparer<V> comparer)
{
return source.Distinct(new CommonEqualityComparer<T, V>(keySelector, comparer));
}
}
借助可选参数,这两个扩展方法也可以合成一个:
public static IEnumerable<T> Distinct<T, V>(this IEnumerable<T> source, Func<T, V> keySelector,
IEqualityComparer<V> comparer = EqualityComparer<V>.Default)
{
return source.Distinct(new CommonEqualityComparer<T, V>(keySelector, comparer));
}
(同样,CommonEqualityComparer<T, V>类的两个构造函数也可以合二为一)
使用示例:
var data3 = new Person[] {
new Person{ ID = 1, Name = "LDP"},
new Person{ ID = 2, Name = "ldp"}
};
var ps3 = data3
.Distinct(p => p.Name, StringComparer.CurrentCultureIgnoreCase)
.ToArray();
EqualityComparer<T> 类 简要说明
EqualityComparer<T>为IEqualityComparer<T> 泛型接口的实现提供基类,它在.net 4 中有五个重要的子类,见下图:
这五个子类分别用不同类型数据的相等性比较,从类名我们可以略知一二。
这五个子类都是内部类(internal),不能直接访问,EqualityComparer<T> 类提供一个简单的属性Default。EqualityComparer<T> 会根据传入的T 的类型,加载不同的子类,并会予以缓存提高性能。
补充:软件开发 , C# ,