c# - 如何将泛型类型参数用作声明为具有类型约束的接口的属性的类型参数？

Question

如何声明和使用泛型接口（请参阅命名空间 Sample2）以与命名空间 Sample1 中的类相同的方式工作？

我知道有一种解决方法（请参阅命名空间 Sample2Modified），但这不是我想要实现的。

---

与类一起工作

使用类时，这很好用：

namespace Sample1
{
    public class ClassContainer<T1, T2>
        where T1 : T2
    { }

    public class ClassParent
    { }

    public class ClassChild : ClassParent
    { }

    public class ClassSampleDoesWork<TItem>
        where TItem : ClassParent
    {
        ClassContainer<IEnumerable<TItem>, IEnumerable<ClassParent>> SomeProperty { get; set; }
    }
}

---

不适用于接口

但是，对于接口，编译器在 ClassSampleDoesNotWork 中声明属性时会发出此警告：

Error 16 The type 'System.Collections.Generic.IEnumerable<TItem>'
cannot be used as type parameter 'T1' in the generic type or method
'Sample2.IInterfaceContainer<T1,T2>'. There is no implicit reference
conversion from 'System.Collections.Generic.IEnumerable<TItem>' to
'System.Collections.Generic.IEnumerable<Sample2.IInterfaceParent>'.

代码：

namespace Sample2
{
    public interface IInterfaceContainer<T1, T2>
        where T1 : T2
    { }

    public interface IInterfaceParent
    { }

    public interface IInterfaceChild : IInterfaceParent
    { }

    public class ClassSampleDoesNotWork<TItem>
        where TItem : IInterfaceParent
    {
        IInterfaceContainer<IEnumerable<TItem>, IEnumerable<IInterfaceParent>> SomeProperty { get; set; }
    }
}

---

修改版有效，但这不是我想要的

如果我将类修改为使用 TEnumerableOfItem 而不是 TItem，它会起作用：

namespace Sample2Modified
{
    public interface IInterfaceContainer<T1, T2>
        where T1 : T2
    { }

    public interface IInterfaceParent
    { }

    public interface IInterfaceChild : IInterfaceParent
    { }

    public class ClassSampleModifiedDoesWork<TEnumerableOfItem>
        where TEnumerableOfItem : IEnumerable<IInterfaceParent>
    {
        IInterfaceContainer<TEnumerableOfItem, IEnumerable<IInterfaceParent>> SomeProperty { get; set; }
    }
}

Answer 1

Try to add class constraint to the TItem:

namespace Sample2
{
    public interface IInterfaceContainer<T1, T2>            
        where T1 : T2
    { }

    public interface IInterfaceParent
    { }

    public interface IInterfaceChild : IInterfaceParent
    { }

    public class ClassSampleDoesNotWork<TItem>
        where TItem : class, IInterfaceParent
    {
        IInterfaceContainer<IEnumerable<TItem>, IEnumerable<IInterfaceParent>> SomeProperty { get; set; }
    }
}

This works because variance only works for reference-types (or there is an identity conversion). It isn't known that TItem is reference type, unless you add : class. Read this article for more information. Here is a sample code to demonstrate this behavior:

IEnumerable<Object> items = new List<int>(); // Compiler error.