C# 中的可选委托 [英] Optional delegates in C#

问题描述

这是两个扩展方法重载的简单示例

This is a simple example of two extension methods overloads

public static class Extended 
{
    public static IEnumerable<int> Even(this List<int> numbers)
    {
        return numbers.Where(num=> num % 2 == 0);
    }

    public static IEnumerable<int> Even(this List<int> numbers, Predicate<int> predicate)
    {
        return numbers.Where(num=> num % 2 == 0 && predicate(num));
    }
}

我希望能够通过将委托设置为可选来将它们合并为一个:

I'd like to be able to merge them into one, by setting a delegate to be optional:

public static class Extended 
{
    public static IEnumerable<int> Even(this List<int> numbers, Predicate<in> predicate = alwaysTrue)
    {
        return numbers.Where(num=> num % 2 == 0 && predicate(num));
    }

    public static bool alwaysTrue(int a) { return true; }
}

然而，编译器抛出一个错误:

However, compiler throws an error:

谓词"的默认参数值必须是编译时常量

Default parameter value for 'predicate' must be a compile-time constant

我不明白我的 alwaysTrue 函数如何不是常量，但是嘿，编译器知道得更好:)

I don't see how my alwaysTrue function is not constant, but hey, compiler knows better :)

有什么办法可以让委托参数成为可选的吗?

Is there any way to make the delegate parameter optional?

推荐答案

它不是常量，因为您从方法组创建了一个委托……就 C# 语言而言，这不是编译时常量.

It's not constant because you've created a delegate from a method group... that's not a compile-time constant as far as the C# language is concerned.

如果你不介意稍微滥用 null 的含义，你可以使用:

If you don't mind abusing the meaning of null slightly you could use:

private static readonly Predicate<int> AlwaysTrue = ignored => true;

public static List<int> Even(this List<int> numbers,
                             Predicate<int> predicate = null)
{
    predicate = predicate ?? AlwaysTrue;
    return numbers.Where(num=> num % 2 == 0 && predicate(num));
}

(您可以仍然使 AlwaysTrue 成为一个方法并使用方法组转换，但是上面的方法通过创建稍微效率更高委托实例只有一次.)

(You could still make AlwaysTrue a method and use a method group conversion, but the above approach is very slightly more efficient by creating the delegate instance just once.)

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