LINQ身份的功能？ [英] LINQ identity function?

问题描述

刚想LINQ语法有点挑剔。我压扁了的IEnumerable< IEnumerable的< T>> 与的SelectMany（X => X）。

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。有一些静态的身份功能的对象，我可以改用的X => X ？ ？像的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

请注意：这个答案是正确的C＃3，但在某些时候（C＃4？C＃5？）类型推断改进，使下图所示的 IdentityFunction 方法的可以的使用容易。

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.

没有，没有。它必须是通用的，先从：

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

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

但随后类型推断是行不通的，所以你必须做

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

SelectMany(Helpers.IdentityFunction<Foo>())

比 X =>难看很多; 。X

另一种可能是您在扩展方法包装这样的：

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);
}

不幸的是与一般的方差事情是这样的，那可能落入各种犯规例如：;在C＃3个案...这将不适用列表<名单，LT;串GT;>。你可以使它更通用的：

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);
}

但同样，你便起身类型推断的问题，我怀疑...

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

编辑：为了应对的意见......是的，C＃4使这更容易。或者说，它使第一个平展办法比它是在C＃3.下面是这在C＃4工作的示例比较有用，但不工作在C＃3，因为编译器不能从列表与LT转换;列表<串>> 到的IEnumerable< IEnumerable的<串>> ：

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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