21扩展IEnumerable<T>泛型接口自定义LINQ的扩展方法
LINQ方法实际上是对IEnumerable<TSource>的扩展,如图:
本篇自定义一个MyWhere方法,达到与Where相同的效果。
使用LINQ自带的Where方法
class Program
{
static void Main(string[] args)
{
List<int> list = new List<int>(){1, 2, 3};
IEnumerable<int> query = list.Where(x => x%2 == 0);
list.Add(4);
showConsole(query);
Console.ReadKey();
}
private static void showConsole<T>(IEnumerable<T> list)
{
foreach (T item in list)
{
Console.WriteLine(item.ToString());
}
}
}
.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }
结果:
这样的结果符合LINQ的"延迟加载"的特点,虽然是在IEnumerable<int> query = list.Where(x => x%2 == 0)之后为集合添加元素list.Add(4),但直到调用showConsole(query)遍历,查询才真正执行。
自定义一个MyWhere,无延迟加载
□ 首先想到的是对IEnumerable<TSource>的扩展,创建静态方法和静态类。
public static class Extension
{
//Func<TSource, bool>是委托,返回的是bool类型
public static IEnumerable<TSource> MyWhere<TSource>(this IEnumerable<TSource> source,
Func<TSource, bool> predicate)
{
if(source==null) throw new ArgumentException();
if(predicate==null) throw new ArgumentException();
List<TSource> result = new List<TSource>();
foreach (TSource item in source)
{
if (predicate(item))
{
result.Add(item);
}
}
return result;
}
}
.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }
□ 执行主程序
class Program
{
static void Main(string[] args)
{
List<int> list = new List<int>(){1, 2, 3};
IEnumerable<int> query = list.MyWhere(x => x % 2 == 0);
list.Add(4);
showConsole(query);
Console.ReadKey();
}
private static void showConsole<T>(IEnumerable<T> list)
{
foreach (T item in list)
{
Console.WriteLine(item.ToString());
}
}
}
.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }
□ 结果
这样的结果丢掉了LINQ的"延迟加载"的特点,也就是后加元素list.Add(4)之后,没有再对集合进行遍历。
可希望的结果是:
● 后加元素list.Add(4)之后,还需要遍历集合
● 返回结果还是IEnumerable<TSource>类型
于是,想到了Decorator设计模式,使用它能满足以上2个条件。
自定义一个MyWhere,也有延迟加载,使用Decorator设计模式
● 为了返回IEnumerable<TSource>类型,必须让装饰者类实现IEnumerable<T>接口
● 装饰者类最重要的特点是包含目标参数类型的引用
● 为了能遍历,装饰者类内部还包含了一个迭代器
public class WhereDecorator<T> : IEnumerable<T>
{
private IEnumerable<T> list;
private Func<T, bool> predicate;
public WhereDecorator(IEnumerable<T> list, Func<T, bool> predicate)
{
this.list = list;
this.predicate = predicate;
}
public IEnumerator<T> GetEnumerator()
{
return new WhereEnumerator<T>(list, predicate);
}
System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
{
return new WhereEnumerator<T>(list, predicate);
}
public class WhereEnumerator<T> : IEnumerator<T>
{
private List<T> innerList;
private int index;
public WhereEnumerator(IEnumerable<T> list, Func<T, bool> predicate)
{
innerList = new List<T>();
index = -1;
foreach (T item in list)
{
if (predicate(item))
{
innerList.Add(item);
}
}
}
public T Current
{
get { return innerList[index]; }
}
public void Dispose()
{
}
object System.Collections.IEnumerator.Current
{
get { return innerList[index]; }
}
public bool MoveNext()
{
index++;
if (index >= innerList.Count)
{
return false;
}
else
{
return true;
}
}
public void Reset()
{
index = -1;
}
}
}
.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }
自定义MyWhere中,现在可以使用装饰者类来返回一个实例。
public static class Extension
{
public static IEnumerable<TSource> MyWhere<TSource>(this IEnumerable<TSource> source,
Func<TSource, bool> predicate)
{
if(source==null) throw new ArgumentException();
if(predicate==null) throw new ArgumentException();
return new WhereDecorator<TSource>(source, predicate);
}
}
.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }
主程序中:
class Program
{
static void Main(string[] args)
{
List<int> list = new List<int>(){1, 2, 3};
IEnumerable<int> query = list.MyWhere(x => x % 2 == 0);
list.Add(4);
showConsole(query);
Console.ReadKey();
}
private static void showConsole<T>(IEnumerable<T> list)
{
foreach (T item in list)
{
Console.WriteLine(item.ToString());
}
}
}
.csharpcode, .csharpcode pre
{
font-size: small;
color: black;
font-family: consolas, "Courier New", courier, monospace;
background-color: #ffffff;
/*white-space: pre;*/
}
.csharpcode pre { margin: 0em; }
.csharpcode .rem { color: #008000; }
.csharpcode .kwrd { color: #0000ff; }
.csharpcode .str { color: #006080; }
.csharpcode .op { color: #0000c0; }
.csharpcode .preproc { color: #cc6633; }
.csharpcode .asp { background-color: #ffff00; }
.csharpcode .html { color: #800000; }
.csharpcode .attr { color: #ff0000; }
.csharpcode .alt
{
background-color: #f4f4f4;
width: 100%;
margin: 0em;
}
.csharpcode .lnum { color: #606060; }
结果:
可见,与LINQ的Where方法返回结果一样。
总结
● 所有的LINQ方法是对IEnumerable<T>的扩展
● 当我们想对方法返回的结果再进行链式操作的时候,装饰者类就包含方法参数类型的引用并返回与该方法相同的类型。
● 这里的装饰者类需要完成遍历,装饰者类必须实现IEnumerable<T>,内部必须存在迭代器实现IEnumerator<T>接口。
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