为什么此方法组转换在 C# 7.2 及更低版本中不明确？

Question

给出以下 class：

public class Foo {
    public Foo(int i, double d) {
        Integer = i;
        Double = d;
    }
    public int Integer {get;}
    public double Double {get;}

    private static Random rand = new Random();
    public static Foo CreateRandom() => new Foo(rand.Next(1,101), rand.NextDouble());
}

还有这个用法：

void Main()
{
    var items = Enumerable.Range(0, 50)
                          .Select(_ => Foo.CreateRandom());

    Console.WriteLine(items.Sum(GetInteger)); // Fine
    Console.WriteLine(items.Sum(GetDouble)); // Ambiguous
    Console.WriteLine(items.Sum(x => x.Double)); // Also fine
    Console.WriteLine(items.Sum((Func<Foo,double>)GetDouble)); // Cast required? Why?

    int GetInteger(Foo item) => item.Integer;
    double GetDouble(Foo item) => item.Double;
}

我想弄清楚为什么 GetDouble 委托转换被认为是不明确的，以及在这种情况下它与 labmda 表达式和对匿名委托的转换的确切区别。

编辑：看起来这不会影响 C# 7.3，但会影响 7.2 及更低版本。添加本地方法之前的版本可能会受到 GetInteger 和 GetDouble 静态化的影响。

Answer 1

C# 7.3 中的新增功能

The following enhancements were made to existing features:

You can test == and != with tuple types.

You can use expression variables in more locations.

You may attach attributes to the backing field of auto-implemented properties.

Method resolution when arguments differ by in has been improved.

Overload resolution now has fewer ambiguous cases.

最后一次修复是针对此问题的。在此之前，编译器在解析重载时遇到了更多困难。

这里是 link 来源。

为什么此方法组转换在 C# 7.2 及更低版本中不明确？

Why is this method group conversion ambiguous in C# 7.2 and lower?

c#

c#-7.0

visual-studio-2017

c#-7.2

c#-7.1