可以返回值和引用类型对象的泛型函数

我有一个helper函数从XML中获取值,它可以很好地处理int和string等值类型。我还有一些类在其构造函数中使用XPathNavigator作为参数,我想执行如下操作:

    public static void SelectSingleNodeSafe<T>(XPathNavigator nav, string pos, out T ret, T def)
    {
        XPathNavigator node = nav.SelectSingleNode(pos);
        if (node != null)
            if (typeof(T).IsSubclassOf(XMLConstructible))
                ret = new T(node);// this won't compile
            else
                ret = (T)node.ValueAs(typeof(T));//this works for my use cases
        else
            ret = def;
    }

有志者事竟成?