c# - 为什么 ReadOnlySpan 不能用作泛型委托(delegate)和泛型方法的类型参数？

Question

我理解为什么 ReadOnlySpan 不能用作泛型类的类型参数。 ReadOnlySpan 只是堆栈，因此它不能用作字段类型，字段成员像它的容器对象一样存在于堆中。然而，返回值和参数始终只是堆栈，那么为什么 ReadOnlySpan 不能用作泛型委托(delegate)和泛型方法的类型参数？

这里有一个例子可以说明我在说什么:

using System;

namespace ConsoleApp7
{
    class Program
    {
        public delegate TResult MyFunc<TResult>(ReadOnlySpan<char> arg);

        static int GetSpanLength(ReadOnlySpan<char> span)
        {
            return span.Length;
        }

        static void Main(string[] args)
        {
            var span = "hello".AsSpan();

            MyFunc<int> func1 = GetSpanLength;
            var result1 = DoSomething(func1, span);

            // The type 'ReadOnlySpan<char>' may not be used as a type argument
            Func<ReadOnlySpan<char>, int> func2 = GetSpanLength;

            //The type 'ReadOnlySpan<char>' may not be used as a type argument
            var result = DoSomething<int, ReadOnlySpan<char>>(func2, span);


        }

        static TResult DoSomething<TResult, T>(Func<T, TResult> func, T arg)
        {
            return func(arg);
        }

        static TResult DoSomething<TResult>(MyFunc<TResult> func, ReadOnlySpan<char> arg)
        {
            return func(arg);
        }

    }
}

这非常不幸，因为它迫使我使用两个相同版本的 DoSomething 方法，使我的代码非常 WET。

注意:对于面向 .NET Framework 的项目，您需要安装 System.Memory Nuget 包。

Answer 1

这post来自 Adam Sitnik 说

Let’s consider following C# code:

Span<byte> Allocate() => new Span<byte>(new byte[256]);

void CallAndPrint<T>(Func<T> valueProvider) // no generic requirements for T
{
    object value = valueProvider.Invoke(); // boxing!

    Console.WriteLine(value.ToString());
}

void Demo()
{
    Func<Span<byte>> spanProvider = Allocate;
    CallAndPrint<Span<byte>>(spanProvider);
}

As you can see the non-boxing requirement can not be ensured today ifwe allow the Span to be generic type argument. One of the possiblesolutions could be to introduce new generic constraint: stackonly. Butthen all the managed compilers would have to respect it and ensure thelack of boxing and other restrictions. This is why it was decided tosimply forbid using Span as a generic argument.