如何解决 IReadOnlyDictionary 缺乏协方差的问题? [英] How to get around lack of covariance with IReadOnlyDictionary?

问题描述

我正在尝试公开一个只读字典，该字典包含具有只读接口的对象.在内部，字典是可写的，其中的对象也是可写的(参见下面的示例代码).我的问题是 IReadOnlyDictionary 不支持协变转换，因为问题中概述的原因 这里.这意味着我不能只将我的内部字典公开为只读.

I'm trying to expose a read-only dictionary that holds objects with a read-only interface. Internally, the dictionary is write-able, and so are the objects within (see below example code). My problem is that IReadOnlyDictionary doesn't support covariant conversions because of the reason outlined in the question here. This means I can't just expose my internal dictionary as a read only one.

所以我的问题是，有没有一种有效的方法可以将我的内部字典转换为 IReadOnlyDictionary，或者其他一些方法来处理这个问题?我能想到的选项是:

So my question is, is there an efficient way to convert my internal dictionary to an IReadOnlyDictionary, or some other way to handle this? The options I can think of are:

1. 持有两个内部词典并保持同步.
2. 在访问属性时创建一个新字典并转换其中的所有对象.
3. 在内部使用时将 IReadOnly 转换回 NotReadOnly.

1 看起来很痛苦，2 看起来效率很低.3 听起来是目前最有前途的，但仍然很难看.我还有其他选择吗?

1 seems like a pain, 2 seems highly inefficient. 3 sounds like the most promising at the moment, but is still ugly. Do I have any other options?

public class ExposesReadOnly
{
    private Dictionary<int, NotReadOnly> InternalDict { get; set; }
    public IReadOnlyDictionary<int, IReadOnly> PublicList
    {
        get
        {
            // This doesn't work...
            return this.InternalDict;
        }
    }

    // This class can be modified internally, but I don't want
    // to expose this functionality.
    private class NotReadOnly : IReadOnly
    {
        public string Name { get; set; }
    }
}

public interface IReadOnly
{
    string Name { get; }
}

推荐答案

您可以为字典编写自己的只读包装器，例如:

You could write your own read-only wrapper for the dictionary, e.g.:

public class ReadOnlyDictionaryWrapper<TKey, TValue, TReadOnlyValue> : IReadOnlyDictionary<TKey, TReadOnlyValue> where TValue : TReadOnlyValue
{
    private IDictionary<TKey, TValue> _dictionary;

    public ReadOnlyDictionaryWrapper(IDictionary<TKey, TValue> dictionary)
    {
        if (dictionary == null) throw new ArgumentNullException("dictionary");
        _dictionary = dictionary;
    }
    public bool ContainsKey(TKey key) { return _dictionary.ContainsKey(key); }

    public IEnumerable<TKey> Keys { get { return _dictionary.Keys; } }

    public bool TryGetValue(TKey key, out TReadOnlyValue value)
    {
        TValue v;
        var result = _dictionary.TryGetValue(key, out v);
        value = v;
        return result;
    }

    public IEnumerable<TReadOnlyValue> Values { get { return _dictionary.Values.Cast<TReadOnlyValue>(); } }

    public TReadOnlyValue this[TKey key] { get { return _dictionary[key]; } }

    public int Count { get { return _dictionary.Count; } }

    public IEnumerator<KeyValuePair<TKey, TReadOnlyValue>> GetEnumerator()
    {
        return _dictionary
                    .Select(x => new KeyValuePair<TKey, TReadOnlyValue>(x.Key, x.Value))
                    .GetEnumerator();
    }

    System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
    {
        return this.GetEnumerator();
    }
}

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