C＃-列表< MyClass>可以吗？无缝转换为List< Interface>或类似的？ [英] C# - Can a List<MyClass> be seamlessly cast to a List<Interface> or similar?

问题描述

我的控件中有一个 DataSource ，它始终是 List< T> ，其中 T 必须从 IEntity 继承。

I have a DataSource in my control which is always a List<T> where T has to inherit from IEntity.

public class MyClass<T> where T : IEntity
{
    public List<T> DataSource
    {
        get;
        set;
    }
}

现在，显然您不能投射 List< T> 到 List< IEntity> 的操作如下：

Now, obviously you can't cast a List<T> to a List<IEntity> doing the following:

List<IEntity> wontWork = (List<IEntity>)this.DataSource;

如何获取数据源作为 IEntity ，同时仍然能够从 DataSource 添加和删除项目？即我可以执行以下操作，但从返回的列表中删除并不会从数据源中删除：

How can I get the DataSource as a List of IEntity, whilst still being able to add and remove items from the DataSource? I.e. I could do the following, but removing from the List it returns would not remove from the DataSource:

public List<TOut> GetDataSourceCopyAsUnderlyingType<TOut>()
{

    if (this.DataSource == null)
    {
        return new List<TOut>();
    }
    else
    {

        // Get the list and the enumerator
        IList list = (IList)this.DataSource;
        IEnumerator enumerator = list.GetEnumerator();

        // Build the target list
        List<TOut> targetList = new List<TOut>();

        int i = 0;
        while (enumerator.MoveNext())
        {
            TOut entity = (TOut)list[i];
            targetList.Add(entity);
            i++;
        }

        return targetList;

        }

    }

基本上，我需要执行以下操作：

Basically, I need some way of doing the following:

List<IEntity> interfaceList = this.GetDataSourceAsAnotherType<IEntity>();
int dataSourceCount = this.DataSource.Count;   // Equals 5
int interfaceCount = interfaceList.Count;      // Equals 5

interfaceList.RemoveAt(0);
int dataSourceCount = this.DataSource.Count;   // Equals 4
int interfaceCount = interfaceList.Count;      // Equals 4

而且，我不介意是否有

编辑：抱歉，忘记了说我正在使用.Net2.0，并且无法移动到.Net 3.5。

EDIT: Sorry, forgot to say I'm using .Net2.0 and cannot move to .Net 3.5.

推荐答案

如果允许这样做将是一个非常糟糕的主意，这就是为什么不允许这样做。我可以将任何旧的IEntity添加到 List< IEntity> 中，如果无法将该IEntity强制转换为T，则会爆炸。尽管所有T是IEntities，但并非所有IEntities都是Ts。

It would be a monumentally bad idea if this were allowed, which is why it isn't. I can add any old IEntity to a List<IEntity> which will blow up if that IEntity can't be cast to T. Whilst all Ts are IEntities, not all IEntities are Ts.

这可用于数组，因为数组有一个故意的子类型化漏洞（就像在Java中一样）。集合没有子类型漏洞。

This works with arrays because arrays have a deliberate subtyping hole (as they do in Java). Collections do not have a subtyping hole.

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