Question

我有两个派生自抽象泛型类的类，它派生自System.Web.UI.Page。我有一个System.Web.UI.MasterPage，可能是也可能不是我的两个派生页面类或System.Web.UI.Page中的任何一个的主。

问题是泛型类有一个我需要在我的MasterPage中访问的属性，但我不知道有任何优雅的方法来获取它。

这是一个例子......

内容类型：

public abstract class Fruit
{
    public int ID { get; set; } //Just an identifier
}

public class Apple : Fruit {  }

public class Banana : Fruit {  }

页数：

public abstract class FruitPage<T> : System.Web.UI.Page where T : Fruit
{
    public T MyFruit { get; set; } 
}

public class ApplePage : FruitPage<Apple> {  }

public class BananaPage : FruitPage<Banana> {  }

主人：

public partial class FoodMaster : System.Web.UI.MasterPage
{
    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);

        if (this.Page is FruitPage<Fruit>) //I know this is wrong
        {
            if ((this.Page as FruitPage<Fruit>).MyFruit.ID <= 0) //This too
            { 
                /*
                    I want to get here (this.Page being an ApplePage or BananaPage).  

                    Basically...  if ID<=0 then it is a new (unsaved) "fruit", 
                        and I need to change some MasterPage GUI accordingly. 
                */
            }
        }
    }
}

谢谢！

Answer 1

"Any problem in computer science can be solved with another level of indirection."

public class FruitPage : System.Web.UI.Page {
   public Fruit MyInnerFruit { get; protected set; }
}

public abstract class FruitPage<T> : FruitPage where T : Fruit
{
   public T MyFruit { 
     get { return (T)MyInnerFruit; } 
     set { MyInnerFruit = value; } 
   }
}

public partial class FoodMaster : System.Web.UI.MasterPage {
    protected override void OnLoad(EventArgs e) {
       base.OnLoad(e);

       var fruitPage = this.Page as FruitPage;
       if (fruitPage != null && fruitPage.MyInnerFruit.ID <= 0) {
          ...
       }
    }
}

你也可以隐藏MyInnerFruit / MyFruit - 但没有它就更容易理解。

如果你使用的是.NET 4，你也可以使用协变界面来基本上允许从FruitPage<Apple>到FruitPage<Fruit>的强制转换：

public interface IFruitPage<out T> where T : Fruit {
    public T MyFruit { get; }
}

public class FruitPage<T> : Page, IFruitPage<T> where T : Fruit {
    public T MyFruit { get; set; }
}

public class FoodMaster : MasterPage {
    protected override void OnLoad(EventArgs e) {
       base.OnLoad(e);

       var fruitPage = this.Page as IFruitPage<Fruit>;
       if (fruitPage != null && fruitPage.MyFruit.ID <= 0) {
          ...
       }
    }
}

MasterPage需要来自System.Web.UI.Page派生的抽象泛型类的属性

1 个答案: