的IList< T>和IReadOnlyList< T>

人气:180 发布:2022-09-21 标签: .net c# interface collections .net-4.5

问题描述

如果我有一个需要参数的方法,

If I have a method that requires a parameter that,

有一个计数 property 有一个整数索引器(get-only) Has a Count property Has an integer indexer (get-only)

该类型应该是什么这个参数是?我会在.NET 4.5之前选择 IList< T> ,因为没有其他可索引的集合接口,并且数组实现它,这是一个很大的优点。

What should the type of this parameter be? I would choose IList<T> before .NET 4.5 since there was no other indexable collection interface for this and arrays implement it, which is a big plus.

但.NET 4.5引入了新的 IReadOnlyList< T> 界面,我也希望我的方法也能支持它。如何在不违反基本原则的情况下编写此方法以支持 IList< T> IReadOnlyList< T> 干?

But .NET 4.5 introduces the new IReadOnlyList<T> interface and I want my method to support that, too. How can I write this method to support both IList<T> and IReadOnlyList<T> without violating the basic principles like DRY?

编辑:Daniel的回答给了我一些想法:

Edit: Daniel's answer gave me some ideas:

public void Foo<T>(IList<T> list)
    => Foo(list, list.Count, (c, i) => c[i]);

public void Foo<T>(IReadOnlyList<T> list)
    => Foo(list, list.Count, (c, i) => c[i]);

private void Foo<TList, TItem>(
    TList list, int count, Func<TList, int, TItem> indexer)
    where TList : IEnumerable<TItem>
{
    // Stuff
}

编辑2 或者我可以接受 IReadOnlyList< T> 并提供如下帮助:

Edit 2: Or I could just accept an IReadOnlyList<T> and provide a helper like this:

public static class CollectionEx
{
    public static IReadOnlyList<T> AsReadOnly<T>(this IList<T> list)
    {
        if (list == null)
            throw new ArgumentNullException(nameof(list));

        return list as IReadOnlyList<T> ?? new ReadOnlyWrapper<T>(list);
    }

    private sealed class ReadOnlyWrapper<T> : IReadOnlyList<T>
    {
        private readonly IList<T> _list;

        public ReadOnlyWrapper(IList<T> list) => _list = list;

        public int Count => _list.Count;

        public T this[int index] => _list[index];

        public IEnumerator<T> GetEnumerator() => _list.GetEnumerator();

        IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
    }
}

然后我可以称之为 Foo(list.AsReadOnly())

编辑3:数组同时实现 IList< T> IReadOnlyList< T> 也是如此列表< T> 类。这使得很难找到一个实现 IList< T> 而不是 IReadOnlyList< T> 的类。

Edit 3: Arrays implement both IList<T> and IReadOnlyList<T>, so does the List<T> class. This makes it pretty rare to find a class that implements IList<T> but not IReadOnlyList<T>.

推荐答案

你在这里运气不好。 IList< T> 未实现 IReadOnlyList< T> 列表< T> 确实实现了两个接口,但我认为这不是你想要的。

You are out of luck here. IList<T> doesn't implement IReadOnlyList<T>. List<T> does implement both interfaces, but I think that's not what you want.

但是,你可以使用LINQ:

However, you can use LINQ:

Count()扩展方法在内部检查实例是否在事实是一个集合,然后使用计数属性。 ElementAt()扩展方法在内部检查实例是否实际上是一个列表而不是使用索引器。 The Count() extension method internally checks whether the instance in fact is a collection and then uses the Count property. The ElementAt() extension method internally checks whether the instance in fact is a list and than uses the indexer.

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