[API Proposal]: ReadOnlySet<T>

[stephentoub]

Background and motivation

If someone wants to return an IList<T> via a read-only wrapper that prevents the list from being mutated via that reference, they can wrap it in a ReadOnlyCollection<T>.

If they have an IDictionary<TKey, TValue>, they can wrap it in a ReadOnlyDictionary<TKey, TValue>.

But if they have an ISet<T>, there's no ReadOnlySet<T> that can be used to easily wrap the set. The dev needs to either write their own wrapper type, and will instead often end up making a copy with an ImmutableHashSet<T> or FrozenSet<T>.

Related:
#76028
#29387
#2293

API Proposal

namespace System.Collections.ObjectModel;

[DebuggerDisplay("Count = {Count}")]
public class ReadOnlySet<T> : IReadOnlySet<T>, ISet<T>, ICollection
{
    public ReadOnlySet(ISet<T> set);

    public static ReadOnlySet<T> Empty { get; }

    protected ISet<T> Set { get; }

    public int Count { get; }
    public IEnumerator<T> GetEnumerator();

    public bool Contains(T item);
    public bool IsProperSubsetOf(IEnumerable<T> other);
    public bool IsProperSupersetOf(IEnumerable<T> other);
    public bool IsSubsetOf(IEnumerable<T> other);
    public bool IsSupersetOf(IEnumerable<T> other);
    public bool Overlaps(IEnumerable<T> other);
    public bool SetEquals(IEnumerable<T> other);

    ... // explicit interface implementations of all other interface methods, throwing from mutating members
}

API Usage

HashSet<int> set = ...;
var ros = new ReadOnlySet<int>(set);

Alternative Designs

• ISet<T> exposes CopyTo while IReadOnlySet<T> does not. ReadOnlySet<T> will implement it, but should it be implicit or explicitly implemented? (Alternatively/separately, should we consider adding CopyTo to IReadOnlyCollection<T> and thus implicitly to IReadOnlySet<T>?)
• Do we also want an ReadOnlySet<T> AsReadOnly() on HashSet<T>? List<T> has ReadOnlyCollection<T> AsReadOnly() and there's a static ReadOnlyCollection<T> AsReadOnly(T[]) on Array, but Dictionary<> doesn't have an AsReadOnly.
• Do we also want an extension method ReadOnlySet<T> AsReadOnly(this ISet<T>)? Such an extension exists for both IList<T> and IDictionary<>.
• What reference assembly should this be exposed from? ReadOnlyCollection/Dictionary are both implemented in CoreLib and exposed via System.Runtime. Should this live there, too? Or in System.Collections.dll?

Risks

n/a

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[N-Olbert]

Should there be an AsReadOnly-(extension)method for HashSet and OrderedSet, like there is one for List?

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

// Or as Extension, probbaly somewhere within System.Linq
public static ReadOnlySet<T> AsReadOnly(this ISet<T> set)
            => new ReadOnlySet<T>(set);

[terrajobst]

Video

• Looks good as proposed

namespace System.Collections.ObjectModel;

[DebuggerDisplay("Count = {Count}")]
public class ReadOnlySet<T> : IReadOnlySet<T>, ISet<T>, ICollection
{
    public ReadOnlySet(ISet<T> set);
    public static ReadOnlySet<T> Empty { get; }
    protected ISet<T> Set { get; }
    public int Count { get; }
    public IEnumerator<T> GetEnumerator();
    public bool Contains(T item);
    public bool IsProperSubsetOf(IEnumerable<T> other);
    public bool IsProperSupersetOf(IEnumerable<T> other);
    public bool IsSubsetOf(IEnumerable<T> other);
    public bool IsSupersetOf(IEnumerable<T> other);
    public bool Overlaps(IEnumerable<T> other);
    public bool SetEquals(IEnumerable<T> other);
}

[KennethHoff]

#29387 was closed because there didn't exist a ReadOnlySet at the time. Now that it exists, will that also come in .Net 9? I didn't see it in the closing PR (#103306).