我有以下代码:

public interface ISomeObject
{
     IList<ISomeObject> Objects { get; }
}
public class SomeObject : ISomeObject
{
    public SomeObject()
    {
        Objects = new List<SomeObject>();
    }
    public List<SomeObject> Objects
    {
         get;
         set;
    }
    IList<ISomeObject> ISomeObject.Objects
    {
        get
        {
            // What to do here?
            // return Objects; // This doesn't work
            return Objects.Cast<ISomeObject>().ToList(); // Works, but creates a copy each time.
         }
    }


SomeObject具有一个公共属性Objects,该属性返回类类型的列表。知道该类类型的客户可以使用它来做他们想要的任何事情。仅了解ISomeObject的客户端只能使用Objects属性来获取IList<ISomeObject>。因为不允许将List<SomeObject>强制转换为IList<ISomeObject>(由于apple and banana issue),所以我需要一种转换方法。使用Cast.ToList()的默认方式有效,但缺点是每次评估属性时都会创建一个新List,这可能会很昂贵。更改ISomeObject.Objects以返回IEnumerable<ISomeObject>的另一缺点是客户端无法再使用索引(这在我的用例中非常相关)。在IEnumerable上重复使用Linq的ElementAt()调用很昂贵。

有谁知道如何避免这两个问题?
(当然,让SomeObject到处都是已知的不是一种选择)。

最佳答案

您可以/应该实现类似于ReadOnlyCollection<T>的类来充当代理。考虑到它将是只读的,因此它可以是“协变的”(不是语言方面的,而是逻辑上的含义,这意味着它可以代理作为TDest的子类/接口的TSource),然后对所有写方法。

像这样的东西(未经测试的代码):

public class CovariantReadOlyList<TSource, TDest> : IList<TDest>, IReadOnlyList<TDest> where TSource : class, TDest
{
    private readonly IList<TSource> source;

    public CovariantReadOlyList(IList<TSource> source)
    {
        this.source = source;
    }

    public TDest this[int index] { get => source[index]; set => throw new NotSupportedException(); }

    public int Count => source.Count;

    public bool IsReadOnly => true;

    public void Add(TDest item) => throw new NotSupportedException();

    public void Clear() => throw new NotSupportedException();

    public bool Contains(TDest item) => IndexOf(item) != -1;

    public void CopyTo(TDest[] array, int arrayIndex)
    {
        // Using the nuget package System.Runtime.CompilerServices.Unsafe
        // source.CopyTo(Unsafe.As<TSource[]>(array), arrayIndex);
        // We love to play with fire :-)

        foreach (TSource ele in source)
        {
            array[arrayIndex] = ele;
            arrayIndex++;
        }
    }

    public IEnumerator<TDest> GetEnumerator() => ((IEnumerable<TDest>)source).GetEnumerator();

    public int IndexOf(TDest item)
    {
        TSource item2 = item as TSource;

        if (ReferenceEquals(item2, null) && !ReferenceEquals(item, null))
        {
            return -1;
        }

        return source.IndexOf(item2);
    }

    public void Insert(int index, TDest item)
    {
        throw new NotSupportedException();
    }

    public bool Remove(TDest item)
    {
        throw new NotSupportedException();
    }

    public void RemoveAt(int index)
    {
        throw new NotSupportedException();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}


像这样使用它:

IList<string> strs = new List<string>();
IList<object> objs = new CovariantReadOlyList<string, object>(strs);

10-08 01:22