当集合被排序时，LINQ 可以使用二分搜索吗?

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问题描述

当我尝试搜索的集合被排序时，我能否以某种方式指示"LINQ 使用二分搜索.我正在使用 ObservableCollection，填充了有序数据，并且我正在尝试使用 Enumerable.First().在我的谓词中，我根据我的集合排序所依据的字段的值进行过滤.

Can I somehow "instruct" LINQ to use binary search when the collection that I'm trying to search is ordered. I'm using an ObservableCollection<T>, populated with ordered data, and I'm trying to use Enumerable.First(<Predicate>). In my predicate, I'm filtering by the value of the field my collection's sorted by.

推荐答案

据我所知，使用内置方法是不可能的.但是，编写一个允许您编写类似内容的扩展方法会相对容易:

As far as I know, it's not possible with the built-in methods. However it would be relatively easy to write an extension method that would allow you to write something like that :

var item = myCollection.BinarySearch(i => i.Id, 42);

(当然，假设您的集合实现了 IList ；如果您不能随机访问项目，则无法执行二分查找)

(assuming, of course, that you collection implements IList ; there's no way to perform a binary search if you can't access the items randomly)

这是一个示例实现:

public static T BinarySearch<T, TKey>(this IList<T> list, Func<T, TKey> keySelector, TKey key)
        where TKey : IComparable<TKey>
{
    if (list.Count == 0)
        throw new InvalidOperationException("Item not found");

    int min = 0;
    int max = list.Count;
    while (min < max)
    {
        int mid = min + ((max - min) / 2);
        T midItem = list[mid];
        TKey midKey = keySelector(midItem);
        int comp = midKey.CompareTo(key);
        if (comp < 0)
        {
            min = mid + 1;
        }
        else if (comp > 0)
        {
            max = mid - 1;
        }
        else
        {
            return midItem;
        }
    }
    if (min == max &&
        min < list.Count &&
        keySelector(list[min]).CompareTo(key) == 0)
    {
        return list[min];
    }
    throw new InvalidOperationException("Item not found");
}

(未测试...可能需要进行一些调整) 现在已测试并修复;)

(not tested... a few adjustments might be necessary) Now tested and fixed ;)

它抛出 InvalidOperationException 的事实可能看起来很奇怪，但这就是 Enumerable.First 在没有匹配项时所做的.

The fact that it throws an InvalidOperationException may seem strange, but that's what Enumerable.First does when there's no matching item.

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