从C#中的基类访问派生类的属性

Question

在C#中,当泛型列表仅包含基类时,访问派生类的属性的最佳方法是什么.

public class ClassA : BaseClass
{
   public object PropertyA { get; set; }
}

public class ClassB: BaseClass
{
    public object PropertyB { get; set; }
}

public class BaseClass
{
}

public void Main
{
    List<BaseClass> MyList = new List<BaseClass>();
    ClassA a = new ClassA();
    ClassB b = new ClassB();

    MyList.Add(a);
    MyList.Add(b);

    for(int i = 0; i < MyList.Count; i++)
    {
        //I would like to access PropertyA abd PropertyB from the derived classes        
    }
}

Answer 1

当然你可以贬低,像这样:

for (int i = 0; i < MyList.Count; i++)
{
    if (MyList[i] is ClassA)
    {
        var a = ((ClassA)MyList[i]).PropertyA;
        // do stuff with a
    }

    if (MyList[i] is ClassB)
    {
        var b = ((ClassB)MyList[i]).PropertyB;
        // do stuff with b
    }
}

...但是,你应该再看一下你想要完成的事情.如果您有需要获取ClassA和ClassB属性的公共代码,那么最好将对这些属性的访问包装到祖先类中的共享,虚拟属性或方法中.

就像是:

public class BaseClass
{
    public virtual void DoStuff() { }
}

public class ClassA : BaseClass
{
    public object PropertyA { get; set; }

    public override void DoStuff() 
    {
        // do stuff with PropertyA 
    }
}

public class ClassB : BaseClass
{
    public object PropertyB { get; set; }

    public override void DoStuff() 
    {
        // do stuff with PropertyB
    }
}

Answer 2

继TimJ的答案之后,您可以编写一个适用于所有类型的扩展方法:

public static IEnumerable<T> OfType<T>(this IEnumerable list)
{
    foreach (var obj in list)
    {
        if (obj is T)
            yield return (T)obj;
    }
}

或者,如果您有Linq,该函数位于命名空间System.Linq中.