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Comment récupérer un élément réel de HashSet <T>?

J'ai lu cette question pourquoi ce n'est pas possible, mais je n'ai pas trouvé de solution au problème.

Je voudrais récupérer un élément d'un .NET HashSet<T> . Je cherche une méthode qui aurait cette signature:

/// <summary>
/// Determines if this set contains an item equal to <paramref name="item"/>, 
/// according to the comparison mechanism that was used when the set was created. 
/// The set is not changed. If the set does contain an item equal to 
/// <paramref name="item"/>, then the item from the set is returned.
/// </summary>
bool TryGetItem<T>(T item, out T foundItem);

Si vous recherchez un élément avec cette méthode dans la série, vous obtiendrez O (1). Le seul moyen de récupérer un élément d'un fichier HashSet<T> est d'énumérer tous les éléments qui sont O (n).

Je n'ai trouvé aucune solution à ce problème autre que de créer mon propre HashSet<T> ou utilisez un Dictionary<K, V>. Une autre idée?

Remarque:
Je ne veux pas vérifier si le HashSet<T> contient l'élément. Je veux obtenir la référence à l'élément qui est stocké dans le HashSet<T> parce que je dois le mettre à jour (sans le remplacer par une autre instance). L'élément que je transmettrais au TryGetItem serait égal (selon le mécanisme de comparaison que j'ai transmis au constructeur), mais ce ne serait pas la même référence.

c#.nethashset
72
Francois C

Ce que vous demandez a été ajouté à . NET Core il y a un an , et était récemment ajouté à .NET 4.7.2 :

Dans .NET Framework 4.7.2, nous avons ajouté quelques types d’API aux types de collection standard qui activeront les nouvelles fonctionnalités comme suit.
- "TryGetValue" est ajouté à SortedSet et à HashSet pour correspondre au modèle Try utilisé dans d’autres types de collection.

La signature est la suivante (trouvée dans .NET 4.7.2 et versions ultérieures):

    //
    // Summary:
    //     Searches the set for a given value and returns the equal value it finds, if any.
    //
    // Parameters:
    //   equalValue:
    //     The value to search for.
    //
    //   actualValue:
    //     The value from the set that the search found, or the default value of T when
    //     the search yielded no match.
    //
    // Returns:
    //     A value indicating whether the search was successful.
    public bool TryGetValue(T equalValue, out T actualValue);

PS .: Si vous êtes intéressé, il y a fonction associée qu'ils ajouteront à l'avenir - HashSet.GetOrAdd (T).

13
Evdzhan Mustafa

C'est en fait une énorme omission dans l'ensemble des collections. Vous aurez besoin d'un dictionnaire de clés uniquement ou d'un hachage permettant de récupérer des références d'objet. Tant de gens l'ont demandé, pourquoi cela ne se règle pas me dépasse.

Sans bibliothèques tierces, la meilleure solution consiste à utiliser Dictionary<T, T> avec des clés identiques aux valeurs, car Dictionary stocke ses entrées sous forme de table de hachage. En termes de performances, il est identique au HashSet, mais il gaspille évidemment de la mémoire (taille d’un pointeur par entrée).

Dictionary<T, T> myHashedCollection;
...
if(myHashedCollection.ContainsKey[item])
    item = myHashedCollection[item]; //replace duplicate
else
    myHashedCollection.Add(item, item); //add previously unknown item
...
//work with unique item
60
JPE

Cette méthode a été ajoutée à . NET Framework 4.7.2 (et . NET Core 2. avant); voir HashSet<T>.TryGetValue . Citer la source :

/// <summary>
/// Searches the set for a given value and returns the equal value it finds, if any.
/// </summary>
/// <param name="equalValue">The value to search for.
/// </param>
/// <param name="actualValue">
/// The value from the set that the search found, or the default value
/// of <typeparamref name="T"/> when the search yielded no match.</param>
/// <returns>A value indicating whether the search was successful.</returns>
/// <remarks>
/// This can be useful when you want to reuse a previously stored reference instead of 
/// a newly constructed one (so that more sharing of references can occur) or to look up
/// a value that has more complete data than the value you currently have, although their
/// comparer functions indicate they are equal.
/// </remarks>
public bool TryGetValue(T equalValue, out T actualValue)
11
Douglas

Qu'en est-il de la surcharge du comparateur d'égalité de chaîne:

  class StringEqualityComparer : IEqualityComparer<String>
{
    public string val1;
    public bool Equals(String s1, String s2)
    {
        if (!s1.Equals(s2)) return false;
        val1 = s1;
        return true;
    }

    public int GetHashCode(String s)
    {
        return s.GetHashCode();
    }
}
public static class HashSetExtension
{
    public static bool TryGetValue(this HashSet<string> hs, string value, out string valout)
    {
        if (hs.Contains(value))
        {
            valout=(hs.Comparer as StringEqualityComparer).val1;
            return true;
        }
        else
        {
            valout = null;
            return false;
        }
    }
}

Et déclarez ensuite le HashSet en tant que:

HashSet<string> hs = new HashSet<string>(new StringEqualityComparer());
5
mp666

Une autre astuce ferait Reflection, en accédant à la fonction interne InternalIndexOf de HashSet. N'oubliez pas que les noms de champs sont codés en dur. Par conséquent, si ceux-ci changent dans les prochaines versions .NET, cela se brisera. La solution suivante ne prend en charge qu'un type spécifique (par exemple, chaîne), mais peut également être générique, je suppose.

public static class Extensions
{
    private static Func<HashSet<string>, string, string> getHashSetInternalValue;

    static Extensions()
    {
        ParameterExpression targetExp = Expression.Parameter(typeof(HashSet<string>), "target");
        ParameterExpression itemExp = Expression.Parameter(typeof(string), "item");

        var slotsExp = Expression.Field(targetExp, typeof(HashSet<string>).GetField("m_slots", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance));

        var indexExp = Expression.Call(targetExp, typeof(HashSet<string>).GetMethod("InternalIndexOf", System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance), itemExp);

        var slotExp = Expression.ArrayAccess(slotsExp, indexExp);
        var valueExp = Expression.Field(slotExp, "value");

        var testExp = Expression.GreaterThanOrEqual(indexExp, Expression.Constant(0));
        var conditionExp = Expression.Condition(testExp, valueExp, Expression.Constant(null, typeof(string)));

        getHashSetInternalValue = Expression.Lambda<Func<HashSet<string>, string, string>>(conditionExp, new[] { targetExp, itemExp }).Compile();
    }

    /// <summary>
    /// Gets the internal item value equal to <paramref name="item"/> or null if <paramref name="item"/> is not contained
    /// </summary>
    public static string GetInternalValue(this HashSet<string> hashet, string item)
    {
        return getHashSetInternalValue(hashet, item);
    }
}

Tester:

var set = new HashSet<string>(StringComparer.OrdinalIgnoreCase) { "TABLE" };
var intern = set.GetInternalValue("table"); 

Console.WriteLine(intern); // prints "TABLE"
3
Lukas

Maintenant, .NET Core 2.0 a cette méthode exacte.

Méthode HashSet.TryGetValue (T, T)

2
P Motameni

Ok, alors, tu peux le faire comme ça

YourObject x = yourHashSet.Where(w => w.Name.Contains("strin")).FirstOrDefault();

Ceci est pour obtenir une nouvelle instance de l'objet sélectionné. Afin de mettre à jour votre objet, vous devez alors utiliser:

yourHashSet.Where(w => w.Name.Contains("strin")).FirstOrDefault().MyProperty = "something";
2
Vulovic Vukasin

SortedSet aurait probablement un temps de recherche O (log n) dans cette circonstance, si l’utilisation était une option. Toujours pas O (1), mais au moins mieux.

1
Erik Dietrich

Implémentation modifiée de @ mp666 answer afin qu'elle puisse être utilisée pour tout type de hachage et permet de remplacer le comparateur d'égalité par défaut.

public interface IRetainingComparer<T> : IEqualityComparer<T>
{
    T Key { get; }
    void ClearKeyCache();
}

/// <summary>
/// An <see cref="IEqualityComparer{T}"/> that retains the last key that successfully passed <see cref="IEqualityComparer{T}.Equals(T,T)"/>.
/// This class relies on the fact that <see cref="HashSet{T}"/> calls the <see cref="IEqualityComparer{T}.Equals(T,T)"/> with the first parameter
/// being an existing element and the second parameter being the one passed to the initiating call to <see cref="HashSet{T}"/> (eg. <see cref="HashSet{T}.Contains(T)"/>).
/// </summary>
/// <typeparam name="T">The type of object being compared.</typeparam>
/// <remarks>This class is thread-safe but may should not be used with any sort of parallel access (PLINQ).</remarks>
public class RetainingEqualityComparerObject<T> : IRetainingComparer<T> where T : class
{
    private readonly IEqualityComparer<T> _comparer;

    [ThreadStatic]
    private static WeakReference<T> _retained;

    public RetainingEqualityComparerObject(IEqualityComparer<T> comparer)
    {
        _comparer = comparer;
    }

    /// <summary>
    /// The retained instance on side 'a' of the <see cref="Equals"/> call which successfully met the equality requirement agains side 'b'.
    /// </summary>
    /// <remarks>Uses a <see cref="WeakReference{T}"/> so unintended memory leaks are not encountered.</remarks>
    public T Key
    {
        get
        {
            T retained;
            return _retained == null ? null : _retained.TryGetTarget(out retained) ? retained : null;
        }
    }


    /// <summary>
    /// Sets the retained <see cref="Key"/> to the default value.
    /// </summary>
    /// <remarks>This should be called prior to performing an operation that calls <see cref="Equals"/>.</remarks>
    public void ClearKeyCache()
    {
        _retained = _retained ?? new WeakReference<T>(null);
        _retained.SetTarget(null);
    }

    /// <summary>
    /// Test two objects of type <see cref="T"/> for equality retaining the object if successful.
    /// </summary>
    /// <param name="a">An instance of <see cref="T"/>.</param>
    /// <param name="b">A second instance of <see cref="T"/> to compare against <paramref name="a"/>.</param>
    /// <returns>True if <paramref name="a"/> and <paramref name="b"/> are equal, false otherwise.</returns>
    public bool Equals(T a, T b)
    {
        if (!_comparer.Equals(a, b))
        {
            return false;
        }

        _retained = _retained ?? new WeakReference<T>(null);
        _retained.SetTarget(a);
        return true;
    }

    /// <summary>
    /// Gets the hash code value of an instance of <see cref="T"/>.
    /// </summary>
    /// <param name="o">The instance of <see cref="T"/> to obtain a hash code from.</param>
    /// <returns>The hash code value from <paramref name="o"/>.</returns>
    public int GetHashCode(T o)
    {
        return _comparer.GetHashCode(o);
    }
}

/// <summary>
/// An <see cref="IEqualityComparer{T}"/> that retains the last key that successfully passed <see cref="IEqualityComparer{T}.Equals(T,T)"/>.
/// This class relies on the fact that <see cref="HashSet{T}"/> calls the <see cref="IEqualityComparer{T}.Equals(T,T)"/> with the first parameter
/// being an existing element and the second parameter being the one passed to the initiating call to <see cref="HashSet{T}"/> (eg. <see cref="HashSet{T}.Contains(T)"/>).
/// </summary>
/// <typeparam name="T">The type of object being compared.</typeparam>
/// <remarks>This class is thread-safe but may should not be used with any sort of parallel access (PLINQ).</remarks>
public class RetainingEqualityComparerStruct<T> : IRetainingComparer<T> where T : struct 
{
    private readonly IEqualityComparer<T> _comparer;

    [ThreadStatic]
    private static T _retained;

    public RetainingEqualityComparerStruct(IEqualityComparer<T> comparer)
    {
        _comparer = comparer;
    }

    /// <summary>
    /// The retained instance on side 'a' of the <see cref="Equals"/> call which successfully met the equality requirement agains side 'b'.
    /// </summary>
    public T Key => _retained;


    /// <summary>
    /// Sets the retained <see cref="Key"/> to the default value.
    /// </summary>
    /// <remarks>This should be called prior to performing an operation that calls <see cref="Equals"/>.</remarks>
    public void ClearKeyCache()
    {
        _retained = default(T);
    }

    /// <summary>
    /// Test two objects of type <see cref="T"/> for equality retaining the object if successful.
    /// </summary>
    /// <param name="a">An instance of <see cref="T"/>.</param>
    /// <param name="b">A second instance of <see cref="T"/> to compare against <paramref name="a"/>.</param>
    /// <returns>True if <paramref name="a"/> and <paramref name="b"/> are equal, false otherwise.</returns>
    public bool Equals(T a, T b)
    {
        if (!_comparer.Equals(a, b))
        {
            return false;
        }

        _retained = a;
        return true;
    }

    /// <summary>
    /// Gets the hash code value of an instance of <see cref="T"/>.
    /// </summary>
    /// <param name="o">The instance of <see cref="T"/> to obtain a hash code from.</param>
    /// <returns>The hash code value from <paramref name="o"/>.</returns>
    public int GetHashCode(T o)
    {
        return _comparer.GetHashCode(o);
    }
}

/// <summary>
/// Provides TryGetValue{T} functionality similar to that of <see cref="IDictionary{TKey,TValue}"/>'s implementation.
/// </summary>
public class ExtendedHashSet<T> : HashSet<T>
{
    /// <summary>
    /// This class is guaranteed to wrap the <see cref="IEqualityComparer{T}"/> with one of the <see cref="IRetainingComparer{T}"/>
    /// implementations so this property gives convenient access to the interfaced comparer.
    /// </summary>
    private IRetainingComparer<T> RetainingComparer => (IRetainingComparer<T>)Comparer;

    /// <summary>
    /// Creates either a <see cref="RetainingEqualityComparerStruct{T}"/> or <see cref="RetainingEqualityComparerObject{T}"/>
    /// depending on if <see cref="T"/> is a reference type or a value type.
    /// </summary>
    /// <param name="comparer">(optional) The <see cref="IEqualityComparer{T}"/> to wrap. This will be set to <see cref="EqualityComparer{T}.Default"/> if none provided.</param>
    /// <returns>An instance of <see cref="IRetainingComparer{T}"/>.</returns>
    private static IRetainingComparer<T> Create(IEqualityComparer<T> comparer = null)
    {
        return (IRetainingComparer<T>) (typeof(T).IsValueType ? 
            Activator.CreateInstance(typeof(RetainingEqualityComparerStruct<>)
                .MakeGenericType(typeof(T)), comparer ?? EqualityComparer<T>.Default)
            :
            Activator.CreateInstance(typeof(RetainingEqualityComparerObject<>)
                .MakeGenericType(typeof(T)), comparer ?? EqualityComparer<T>.Default));
    }

    public ExtendedHashSet() : base(Create())
    {
    }

    public ExtendedHashSet(IEqualityComparer<T> comparer) : base(Create(comparer))
    {
    }

    public ExtendedHashSet(IEnumerable<T> collection) : base(collection, Create())
    {
    }

    public ExtendedHashSet(IEnumerable<T> collection, IEqualityComparer<T> comparer) : base(collection, Create(comparer))
    {
    }

    /// <summary>
    /// Attempts to find a key in the <see cref="HashSet{T}"/> and, if found, places the instance in <paramref name="original"/>.
    /// </summary>
    /// <param name="value">The key used to search the <see cref="HashSet{T}"/>.</param>
    /// <param name="original">
    /// The matched instance from the <see cref="HashSet{T}"/> which is not neccessarily the same as <paramref name="value"/>.
    /// This will be set to null for reference types or default(T) for value types when no match found.
    /// </param>
    /// <returns>True if a key in the <see cref="HashSet{T}"/> matched <paramref name="value"/>, False if no match found.</returns>
    public bool TryGetValue(T value, out T original)
    {
        var comparer = RetainingComparer;
        comparer.ClearKeyCache();

        if (Contains(value))
        {
            original = comparer.Key;
            return true;
        }

        original = default(T);
        return false;
    }
}

public static class HashSetExtensions
{
    /// <summary>
    /// Attempts to find a key in the <see cref="HashSet{T}"/> and, if found, places the instance in <paramref name="original"/>.
    /// </summary>
    /// <param name="hashSet">The instance of <see cref="HashSet{T}"/> extended.</param>
    /// <param name="value">The key used to search the <see cref="HashSet{T}"/>.</param>
    /// <param name="original">
    /// The matched instance from the <see cref="HashSet{T}"/> which is not neccessarily the same as <paramref name="value"/>.
    /// This will be set to null for reference types or default(T) for value types when no match found.
    /// </param>
    /// <returns>True if a key in the <see cref="HashSet{T}"/> matched <paramref name="value"/>, False if no match found.</returns>
    /// <exception cref="ArgumentNullException">If <paramref name="hashSet"/> is null.</exception>
    /// <exception cref="ArgumentException">
    /// If <paramref name="hashSet"/> does not have a <see cref="HashSet{T}.Comparer"/> of type <see cref="IRetainingComparer{T}"/>.
    /// </exception>
    public static bool TryGetValue<T>(this HashSet<T> hashSet, T value, out T original)
    {
        if (hashSet == null)
        {
            throw new ArgumentNullException(nameof(hashSet));
        }

        if (hashSet.Comparer.GetType().IsInstanceOfType(typeof(IRetainingComparer<T>)))
        {
            throw new ArgumentException($"HashSet must have an equality comparer of type '{nameof(IRetainingComparer<T>)}' to use this functionality", nameof(hashSet));
        }

        var comparer = (IRetainingComparer<T>)hashSet.Comparer;
        comparer.ClearKeyCache();

        if (hashSet.Contains(value))
        {
            original = comparer.Key;
            return true;
        }

        original = default(T);
        return false;
    }
}
1
Graeme Wicksted