C＃lambda表达式和的IComparer [英] C# lambda expressions and IComparer

问题描述

我使用lambda表达式进行排序和搜索在C＃中的数组。我不希望实现我的课IComparer接口，因为我需要进行排序，并在多个成员字段进行搜索。

I am using lambda expressions to sort and search an array in C#. I don't want to implement the IComparer interface in my class, because I need to sort and search on multiple member fields.

class Widget
{
    public int foo;

    public void Bar()
    {
        Widget[] widgets;

        Array.Sort(widgets, (a, b) => a.foo.CompareTo(b.foo));

        Widget x = new Widget();
        x.foo = 5;
        int index = Array.BinarySearch(widgets, x,
                                       (a, b) => a.foo.CompareTo(b.foo));
    }
}

虽然排序工作正常，二进制搜索给出了一个编译错误的无法转换lambda表达式到类型System.Collections.IComparer<窗口小部件>'，因为它不是一个委托类型的。出于某种原因，排序中同时包含的IComparer和比较过载，而二分查找只支持IComparer的。经过一番研究，我发现了笨重的 ComparisonComparer< T> 来比较到的IComparer转换：

While the sort works fine, the binary search gives a compilation error Cannot convert lambda expression to type 'System.Collections.IComparer<Widget>' because it is not a delegate type. For some reason, Sort has overloads for both IComparer and Comparison, but BinarySearch only supports IComparer. After some research, I discovered the clunky ComparisonComparer<T> to convert the Comparison to an IComparer:

public class ComparisonComparer<T> : IComparer<T>
{
    private readonly Comparison<T> comparison;

    public ComparisonComparer(Comparison<T> comparison)
    {
        this.comparison = comparison;
    }

    int IComparer<T>.Compare(T x, T y)
    {
        return comparison(x, y);
    }
}

这允许二进制搜索的工作方式如下：

This allows the binary search to work as follows:

int index = Array.BinarySearch(
  widgets,
  x,
  new ComparisonComparer<Widget>((a, b) => a.foo.CompareTo(b.foo)));

呸。是否有一个更清洁的方式？

Yuck. Is there a cleaner way?

推荐答案

好了，一个选项是创建类似 ProjectionComparer 来代替。我有在 MiscUtil 一个版本 - 它基本上是创建一个的IComparer< T> 从投影

Well, one option is to create something like ProjectionComparer instead. I've got a version of that in MiscUtil - it basically creates an IComparer<T> from a projection.

所以，你的例子是：

int index = Array.BinarySearch(widgets, x,
                               ProjectionComparer<Widget>.Create(x => x.foo));

或者你可以实施 T [] 做同样的事情：

public static int BinarySearchBy<TSource, TKey>(
    this TSource[] array,
    TSource value,
    Func<TSource, TKey> keySelector)
{
    return Array.BinarySearch(array, value,
                              ProjectionComparer.Create(array, keySelector));
}

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