c# - 为什么 Enumerable.Cast<> 不使用我的转换运算符？

Question

使用这种类型:

class Foo
{
  public static implicit operator int(Foo obj)
  {
    return 5;
  }
}

var test=new[] { new Foo() };

以下按预期工作

var ok=test.Select(x => (int)x).ToList();

但使用 Cast<> 失败并出现 InvalidCastException - 为什么？

var fail=test.Cast<int>().ToList();

Answer 1

阅读 Jon Skeet 关于重新实现 Linq (EduLinq) 的博客，特别是 part 33 ，他是这样说的:

It's worth noting that (as of .NET 3.5 SP1) Cast and OfType only perform reference and unboxing conversions. They won't convert a boxed int to a long, or execute user-defined conversions. Basically they follow the same rules as converting from object to a generic type parameter. (That's very convenient for the implementation!)