c# - 高效返回 IList来自列表（避免从列表中投射列出）

Question

标签： c#interfacecasting

Answer 1

您可以/应该实现一个类似于ReadOnlyCollection<T>充当代理的类。考虑到它是只读的，它可以是“协变的”（不是语言方面的，而是逻辑上的，这意味着它可以代理 aTDest作为 的子类/接口TSource），然后throw NotSupportedException()是所有的 write 方法。

像这样的东西（代码未经测试）：

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);