How to get the actual item from a HashSet <T>?

Question

I read this question about why this is not possible, but did not find a solution to the problem.

I would like to get an element from a .NET HashSet<T> . I am looking for a method that will have this 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); 

The set search for an element with this method will be O (1). The only way to get an element from a HashSet<T> is to list all the elements that are O (n).

I did not find any workaround for this problem, except for my creation of HashSet<T> or using Dictionary<K, V> . Any other idea?

Note:
I do not want to check if the HashSet<T> element contains an element. I want to get a link to an element that is stored in a HashSet<T> because I need to update it (without replacing it with another instance). The element that I would pass to TryGetItem would be equal (according to the comparison mechanism that I passed to the constructor), but that would not be the same link.

Answer 1

What you requested was added to .NET Core a year ago and recently added to .NET 4.7.2 :

  In .NET Framework 4.7.2 we have added a few APIs to the standard Collection types that will enable new functionality as follows. 'TryGetValue' is added to SortedSet and HashSet to match the Try pattern used in other collection types. 

The signature is as follows (found in .NET 4.7.2 and later):

  // // 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 .: In case you are interested, there is a function that they will add in the future - HashSet.GetOrAdd (T).

Answer 2

This is truly a huge omission in the collection. You only need a key dictionary or HashSet, which allows you to search for references to objects. So many people have asked for this, why he is not correcting, is outside of me.

Without third-party libraries, the best solution is to use Dictionary<T, T> with keys identical to the values, since the Dictionary stores its entries as a hash table. In terms of performance, this is the same as a HashSet, but it naturally spends memory (the size of a write pointer).

 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 

Answer 3

This method was added in the .NET Framework 4.7.2 (and the .NET Core 2.0 before it); see HashSet<T>.TryGetValue . Referring to the 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) 

Answer 4

What about the string equality matching overload:

  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; } } } 

And then declare the HashSet as:

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

Answer 5

Reflection will do another trick by referring to the internal function InternalIndexOf HashSet. Keep in mind that field names are hard-coded, so if these changes are violated in future versions of .NET. The following solution only supports one specific type (e.g. string), but can also be made generic, I think.

 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); } } 

Test:

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

Answer 6

Now .NET Core 2.0 has this exact method.

HashSet.TryGetValue (T, T) Method

Answer 7

SortedSet will probably have an O (log n) lookup time in this case if using this parameter. Still not O (1), but at least better.

Answer 8

So you can do it like this:

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

This is to get a new instance of the selected object. To update your object, you should use:

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

Answer 9

A modified implementation of @ mp666's answer, so it can be used for any type of HashSet and allows you to override the default comparative mapper.

 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}"/> 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; } } 

Answer 10

HashSet has a Contains (T) method.

You can specify IEqualityComparer if you need a custom comparison method (for example, save a person object, but use an SSN to compare equality).

Answer 11

You can also use the ToList () method and apply an index to it.

 HashSet<string> mySet = new HashSet(); mySet.Add("mykey"); string key = mySet.toList()[0];