LINQ 身份函数 [英] LINQ identity function

问题描述

对 LINQ 语法有点小题大做.我正在使用 SelectMany(x => x) 将 IEnumerable> 展平.

Just a little niggle about LINQ syntax. I'm flattening an IEnumerable<IEnumerable<T>> with SelectMany(x => x).

我的问题是 lambda 表达式 x =>x.它看起来有点丑.是否有一些静态的身份函数"对象可以代替 x => 使用?x?类似于 SelectMany(IdentityFunction)?

My problem is with the lambda expression x => x. It looks a bit ugly. Is there some static 'identity function' object that I can use instead of x => x? Something like SelectMany(IdentityFunction)?

推荐答案

注意:这个答案对于 C# 3 是正确的，但是在某些时候(C# 4?C# 5?)类型推断得到了改进，因此 IdentityFunction<下面显示的/code> 方法很容易使用.

Note: this answer was correct for C# 3, but at some point (C# 4? C# 5?) type inference improved so that the IdentityFunction method shown below can be used easily.

不，没有.它必须是通用的，首先是:

No, there isn't. It would have to be generic, to start with:

public static Func<T, T> IdentityFunction<T>()
{
    return x => x;
}

但是类型推断不起作用，所以你必须这样做:

But then type inference wouldn't work, so you'd have to do:

SelectMany(Helpers.IdentityFunction<Foo>())

这比 x => 丑多了x.

另一种可能性是您将其包装在扩展方法中:

Another possibility is that you wrap this in an extension method:

public static IEnumerable<T> Flatten<T>
    (this IEnumerable<IEnumerable<T>> source)
{
    return source.SelectMany(x => x);
}

不幸的是，它的通用变化方式很可能与 C# 3 中的各种情况发生冲突……例如，它不适用于 List>.你可以让它更通用:

Unfortunately with generic variance the way it is, that may well fall foul of various cases in C# 3... it wouldn't be applicable to List<List<string>> for example. You could make it more generic:

public static IEnumerable<TElement> Flatten<TElement, TWrapper>
    (this IEnumerable<TWrapper> source) where TWrapper : IEnumerable<TElement>
{
    return source.SelectMany(x => x);
}

但是，你又遇到了类型推断问题，我怀疑...

But again, you've then got type inference problems, I suspect...

回复评论...是的，C# 4 使这更容易.或者更确切地说，它使第一个 Flatten 方法比在 C# 3 中更有用.这是一个在 C# 4 中有效但在 C# 3 中不起作用的示例，因为编译器无法从List>到IEnumerable>:

To respond to the comments... yes, C# 4 makes this easier. Or rather, it makes the first Flatten method more useful than it is in C# 3. Here's an example which works in C# 4, but doesn't work in C# 3 because the compiler can't convert from List<List<string>> to IEnumerable<IEnumerable<string>>:

using System;
using System.Collections.Generic;
using System.Linq;

public static class Extensions
{
    public static IEnumerable<T> Flatten<T>
        (this IEnumerable<IEnumerable<T>> source)
    {
        return source.SelectMany(x => x);
    }
}

class Test
{
    static void Main()
    {
        List<List<string>> strings = new List<List<string>>
        {
            new List<string> { "x", "y", "z" },
            new List<string> { "0", "1", "2" }
        };

        foreach (string x in strings.Flatten())
        {
            Console.WriteLine(x);
        }
    }
}

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