c# - 为什么将单个元素与 IEnumerable 进行比较不是编译错误

Question

var fooRef = new FooRef();
var fooRefEnumerable = Enumerable.Empty<FooRef>();

var fooRefEquality = (fooRef == fooRefEnumerable); //This compiles without any errors

var fooVal = new FooVal();
var fooValEnumerable = Enumerable.Empty<FooVal>();

//Compilation error : Error 1 Operator '==' cannot be applied to operands of type 'FooVal' and 'System.Collections.Generic.IEnumerable<FooVal>'
var fooValEquality = (fooVal == fooValEnumerable); 

public class FooRef { }
public struct FooVal { }   

为什么要比较单个对象和对 RefTypes 有效的 IEnumerable？

Answer 1

Why is it comparing a single object and an IEnumerable is valid for RefTypes?

因为返回true是完全可行的:

class CunningFooRef : FooRef, IEnumerable<FooRef>
{
    // Implementation...
}

FooRef fooRef = new CunningFooRef();
IEnumerable<FooRef> fooRefEnumerable = Enumerable.Empty<FooRef>();
Console.WriteLine(fooRef == fooRefEnumerable); // False

fooRefEnumerable = (IEnumerable<FooRef>) fooRef;
Console.WriteLine(fooRef == fooRefEnumerable); // True

请注意，这对 fooRef 使用相同的编译时类型和 fooRefEnumerable如您所用。

如果封FooRef ，因此编译器知道对于 FooRef 来说不可能引用也是一个 IEnumerable<FooRef>引用，那么你会得到一个编译时错误。