c# - 为什么我不能将 'as' 与被限制为接口(interface)的泛型类型参数一起使用？

Question

在下面的示例中(仅用于演示目的)，如果 T 不受类约束，则此转换:

var ret = objectA as T;

..将导致以下编译错误:

The type parameter 'T' cannot be used with the 'as' operator because it does not have a class type constraint nor a 'class' constraint.

我不明白为什么我不能这样做。由于我已将 T 约束为接口(interface) IObject，因此编译器应该知道 T 必须是接口(interface)类型并且 as 操作应该是有效的。

public interface IObject
{
    string Id { get; set; }
}
public class ObjectA : IObject
{
    public string Id { get; set; }
}
public class ObjectFactory
{
    public T CreateObject<T>(string id) where T : IObject
    {
        ObjectA objectA = new ObjectA();
        var x = objectA as IObject; // this is good
        var ret = objectA as T; // why this 'as' cannot compile?
        return ret;
    }
    public T CreateClassObject<T>(string id) where T : class, IObject
    {
        ObjectA objectA = new ObjectA();
        var ret = objectA as T; // if T is class, this 'as' can compile
        return ret;
    }
}

Answer 1

since I have constrained T to be interface IObject, compiler should know that T must be an interface type and the 'as' operation should be valid.

不，T不必是接口(interface)类型。它必须是实现接口(interface)的类型。考虑:

public struct Foo : IObject
{
    public string Id { get; set; }
}

现在你会期待什么CreateObject<Foo>("ff")做什么？

随着 class对 CreateObject 的约束，该调用将无效，因为 Foo不是引用类型——编译器知道 T是引用类型，所以 objectA as T没关系。