如何铸造泛型<；T>；到Generic<；R>；其中T是R的一个子类

有没有一种方法可以定义显式转换器或类似的东西，使编译器可以从Generic<R> where T : R自动填充Generic<T>？

我得到的错误信息是：

值"Reference 1[Dog]" is not of type "Reference 1[Pet]"不能在此泛型集合中使用。

我的参考类如下：

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
}