C# HashSet<T> 只读解决方法

Question

这是示例代码：

static class Store
{
    private static List<String> strList = new List<string>();
    private static HashSet<String> strHashSet = new HashSet<string>();

    public static List<String> NormalList
    {
        get { return strList; }
    }

    public static HashSet<String> NormalHashSet
    {
        get { return strHashSet; }
    }

    public static IReadOnlyList<String> ReadonlyList
    {
        get { return (IReadOnlyList<String>)strList; }
    }

    public static IReadOnlyCollection<String> ReadonlyHashSet
    {
        get { return (IReadOnlyCollection<String>)strHashSet; }
    }

    public static IReadOnlyList<String> Real_ReadonlyList
    {
        get { return (IReadOnlyList<String>)strList.AsReadOnly(); }
    }

    public static IReadOnlyCollection<String> Real_ReadonlyHashSet
    {
        get
        {
            List<String> tmpList = new List<String>(strHashSet);
            return (IReadOnlyList<String>)(tmpList).AsReadOnly();
        }
    }
}

这是一个测试代码：

// normal behaviour
// you can modify the list and the hashset

Store.NormalList.Add("some string 1");

Store.NormalHashSet.Add("some string 1");

// tricky behaviour
// you can still modify the list and the hashset

((List<String>)Store.ReadonlyList).Add("some string 2");

((HashSet<String>)Store.ReadonlyHashSet).Add("some string 2");

// expected read-only behaviour
// you can NOT modify

// throws InvalidCastException
((List<String>)Store.Real_ReadonlyList).Add("some string 3");
// throws InvalidCastException
((HashSet<String>)Store.Real_ReadonlyHashSet).Add("some string 3");

我的问题是：

"Real_ReadonlyHashSet" 属性?

有没有更好的解决方案微软有一天会实现 HashSet 的 "AsReadOnly" 方法吗？

Answer 1

这里是 the entirety of the code，共 .AsReadOnly()

public ReadOnlyCollection<T> AsReadOnly() {
    Contract.Ensures(Contract.Result<ReadOnlyCollection<T>>() != null);
    return new ReadOnlyCollection<T>(this);
}

如果您不使用 CodeContracts，甚至不需要第一行。但是，ReadOnlyCollection<T> 只支持 IList<T> 而 HashSet<T> 不支持。

我会做的是制作你自己的 ReadOnlySet<T> class 接受 ISet<T> 并且只通过读取操作 like ReadOnlyCollection<T> does internally.

更新： 这是一个完全充实的 ReadOnlySet<T> 我很快写了一个扩展方法，该方法将 .AsReadOnly() 添加到任何实现 ISet<T>

的东西上

public static class SetExtensionMethods
{
    public static ReadOnlySet<T> AsReadOnly<T>(this ISet<T> set)
    {
        return new ReadOnlySet<T>(set);
    }
}

public class ReadOnlySet<T> : IReadOnlyCollection<T>, ISet<T>
{
    private readonly ISet<T> _set;
    public ReadOnlySet(ISet<T> set)
    {
        _set = set;
    }

    public IEnumerator<T> GetEnumerator()
    {
        return _set.GetEnumerator();
    }

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

    void ICollection<T>.Add(T item)
    {
        throw new NotSupportedException("Set is a read only set.");
    }

    public void UnionWith(IEnumerable<T> other)
    {
        throw new NotSupportedException("Set is a read only set.");
    }

    public void IntersectWith(IEnumerable<T> other)
    {
        throw new NotSupportedException("Set is a read only set.");
    }

    public void ExceptWith(IEnumerable<T> other)
    {
        throw new NotSupportedException("Set is a read only set.");
    }

    public void SymmetricExceptWith(IEnumerable<T> other)
    {
        throw new NotSupportedException("Set is a read only set.");
    }

    public bool IsSubsetOf(IEnumerable<T> other)
    {
        return _set.IsSubsetOf(other);
    }

    public bool IsSupersetOf(IEnumerable<T> other)
    {
        return _set.IsSupersetOf(other);
    }

    public bool IsProperSupersetOf(IEnumerable<T> other)
    {
        return _set.IsProperSupersetOf(other);
    }

    public bool IsProperSubsetOf(IEnumerable<T> other)
    {
        return _set.IsProperSubsetOf(other);
    }

    public bool Overlaps(IEnumerable<T> other)
    {
        return _set.Overlaps(other);
    }

    public bool SetEquals(IEnumerable<T> other)
    {
        return _set.SetEquals(other);
    }

    public bool Add(T item)
    {
        throw new NotSupportedException("Set is a read only set.");
    }

    public void Clear()
    {
        throw new NotSupportedException("Set is a read only set.");
    }

    public bool Contains(T item)
    {
        return _set.Contains(item);
    }

    public void CopyTo(T[] array, int arrayIndex)
    {
        _set.CopyTo(array, arrayIndex);
    }

    public bool Remove(T item)
    {
        throw new NotSupportedException("Set is a read only set.");
    }

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

    public bool IsReadOnly
    {
        get { return true; }
    }
}

Answer 2

您可以编写自己的 IReadOnlyCollection<T> 实现，它包含一个 IEnumerable<T> 和一个计数：

public sealed class ReadOnlyCollectionFromEnumerable<T>: IReadOnlyCollection<T>
{
    readonly IEnumerable<T> _data;

    public ReadOnlyCollectionFromEnumerable(IEnumerable<T> data, int count)
    {
        _data = data;
        Count = count;
    }

    public IEnumerator<T> GetEnumerator()
    {
        return _data.GetEnumerator();
    }

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

    public int Count { get; }
}

然后你像这样声明你的 ReadonlyHashSet 属性：

public static IReadOnlyCollection<String> ReadonlyHashSet
{
    get { return new ReadOnlyCollectionFromEnumerable<string>(strHashSet, strHashSet.Count); }
}

我认为这会解决问题。

Answer 3

HashSet implements the IReadOnlyCollection interface starting with the .NET Framework 4.6; in previous versions of the .NET Framework, the HashSet class did not implement this interface.

Read in docs.microsoft.com

Answer 4

在 .NET Framework 4.6 版本中，HashSet 实现了 IReadOnlyCollection 接口和 ISet 接口。 link... 好像是。

您也可以这样做，但可能会影响性能：

var foo = (IReadOnlyCollection<string>) mySet.toList();

Answer 5

从 .NET 5 开始，字典的 HashSet<T> class now implements the IReadOnlySet<T> interface. There is no built-in ReadOnlySet<T> wrapper though, analogous to the existing ReadOnlyDictionary<TKey, TValue> class，但实现一个是微不足道的：

public class ReadOnlySet<T> : IReadOnlySet<T>
{
    private readonly ISet<T> _set;
    public ReadOnlySet(ISet<T> set) { ArgumentNullException.ThrowIfNull(set); _set = set; }

    public int Count => _set.Count;
    public bool Contains(T item) => _set.Contains(item);
    public bool IsProperSubsetOf(IEnumerable<T> other) => _set.IsProperSubsetOf(other);
    public bool IsProperSupersetOf(IEnumerable<T> other) => _set.IsProperSupersetOf(other);
    public bool IsSubsetOf(IEnumerable<T> other) => _set.IsSubsetOf(other);
    public bool IsSupersetOf(IEnumerable<T> other) => _set.IsSupersetOf(other);
    public bool Overlaps(IEnumerable<T> other) => _set.Overlaps(other);
    public bool SetEquals(IEnumerable<T> other) => _set.SetEquals(other);
    public IEnumerator<T> GetEnumerator() => _set.GetEnumerator();
    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

即将推出的 .NET 7 也将为 IDictionary<TKey,TValue>s 提供一个新的 AsReadOnly 扩展方法，所以让我们也为 ISet<T>s 做一个：

public static ReadOnlySet<T> AsReadOnly<T>(this ISet<T> set) => new ReadOnlySet<T>(set);

用法示例：

HashSet<Item> items = new();
ReadOnlySet<Item> readOnlyItems = items.AsReadOnly();

C# HashSet<T> 只读解决方法

C# HashSet<T> read-only workaround

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