c# - 如何将 Generic 转换为 Generic，其中 T 是 R 的子类？

Question

有没有办法定义一个显式转换器或任何类似的东西，以便编译器可以自动填充 Generic<T>来自Generic<R> where T : R ？

我得到的错误信息是:

The value "Reference1[Dog]" is not of type "Reference1[Pet]" and cannot be used in this generic collection.

我的引用类是这样的:

public interface IReference<T>
{
    T Value { get; }
}

public class Reference<T> : IReference<T>
{
    #region Properties
    public string ref_type { get; set; }
    public string ref_id { get; set; }
    public Uri href { get; set; }

    private bool resolved = false;
    private T _value = default(T);

    public T Value
    { 
        get
        {
            if( !resolved && href != null ) resolve();
            else if( href == null ) return null;
            return _value;
        }
    }
    #endregion

    #region Constructors
    public Reference() { }
    public Reference(string ref_id) 
    {
        this.ref_id = ref_id;
    }
    #endregion

    #region members
    private void resolve()
    {
        if( href != null )
        {
            _value = APIRequestMethods.GetUri<T>(href);
            resolved = true;
        }
        else
            throw new Exception("Cannot resolve the reference because the href property has not been set.");
    }
    #endregion
}

Answer 1

只有当你的泛型类型是一个接口(interface)，并且你让你的泛型类型协变时，这才有效，即:IGeneric<out T> .

请注意，这仅适用于接口(interface)或委托(delegate)。有关详细信息，请参阅 Covariance and Contravariance in Generics .