如何使用Linq / C＃在当前和当前列表中获取项目？ [英] How to get the item before current and after current in a list with Linq / C#?

问题描述

我有一个项目清单，如果我选择一个项目，然后我会给出一个选项上一个和下一个。我已经添加了一个快速的代码示例，但我希望有一个更好/更快的方法来做到这一点，例如500个项目。

有没有linq选项或者什么的？

我已经检查了enumarator，但只有一个moveNext不能设置当前。

快速示例：

项目是一个字典

项目是字典中的一个keyvaluepair

  var match = false; 
 var save = new KeyValuePair< ExtendedProjectLightPlan，Page>（）; 
 var before = new KeyValuePair< ExtendedProjectLightPlan，Page>（）; 
 var after = new KeyValuePair< ExtendedProjectLightPlan，Page>（）; 
 foreach（var p在项目中）
 {
 before = save; 
 save = p; 
 
 if（match）
 {
 after = p; 
 break; 
 
 
 if（p.Key.Id == project.Key.Id）
 {
 match = true; 
 
 
 $ / code $ / $ p 
 
解决方案
没有什么内置的LINQ来做到这一点，但你可以很容易地编写自己的...这是一个实现，它使用.NET 4中的 Tuple 。它将返回n-2个项目对于最初有n个项目的序列，但是如果需要可以调整。 
 
 
  public IEnumerable （这个IEnumerable< T>源）
 {
 //实际上产生前两个以及当前的 - 这个
 //比上一个和下一个，但它们相当于使用（IEnumerator< T> iterator = source.GetEnumerator（））
 {
 if（！iterator.MoveNext（））
 { 
收益率突破; 
} 
 T lastButOne = iterator.Current; 
 if（！iterator.MoveNext（））
 {
 yield break; 
} 
 T previous = iterator.Current; 
 while（iterator.MoveNext（））
 {
 T current = iterator.Current; 
产量返回Tuple.Create（lastButOne，previous，current）; 
 lastButOne =上一个; 
上一个=当前; 
 
 
 
 
 
 
 $ b 
请注意，根据LukeH的回答，字典是无序的...但希望上述将帮助你。
I have a list of projects and if i select a project then i will give a option previous and next. I have add a quick code example but i hope there is a better / faster way to do this for example 500 projects. 

Is there maybe a linq option or something ?

I have checked enumarator but that haves only a moveNext en can't set the current.

Quick example:

projects is a dictionary 

project is a keyvaluepair that exists in the dictionary
var match = false;
var save = new KeyValuePair<ExtendedProjectLightPlan, Page>();
var before = new KeyValuePair<ExtendedProjectLightPlan, Page>();
var after = new KeyValuePair<ExtendedProjectLightPlan, Page>();
foreach (var p in projects)
{
    before = save;
    save = p;

    if (match)
    {
        after = p;
        break;
    }

    if (p.Key.Id == project.Key.Id)
    {
        match = true;
    }                
}

解决方案
There's nothing built into LINQ to do this, but you could write your own fairly easily... here's an implementation which uses Tuple from .NET 4. It will return n-2 items for a sequence which originally has n items - but you could adjust that if necessary.
public IEnumerable<Tuple<T, T, T>> WithNextAndPrevious<T>
    (this IEnumerable<T> source)
{
    // Actually yield "the previous two" as well as the current one - this
    // is easier to implement than "previous and next" but they're equivalent
    using (IEnumerator<T> iterator = source.GetEnumerator())
    {
        if (!iterator.MoveNext())
        {
            yield break;
        }
        T lastButOne = iterator.Current;
        if (!iterator.MoveNext())
        {
            yield break;
        }
        T previous = iterator.Current;
        while (iterator.MoveNext())
        {
            T current = iterator.Current;
            yield return Tuple.Create(lastButOne, previous, current);
            lastButOne = previous;
            previous = current;
        }
    }        
}
Note that as per LukeH's answer, dictionaries are unordered... but hopefully the above will help you anyway.

                        
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