如何从 HashSet < T > 检索实际项？

我读过 这个问题关于为什么这是不可能的，但是还没有找到解决这个问题的方法。

我希望从。NET HashSet<T>.我正在寻找一种方法，它会有这样的签名:

/// <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);

使用这种方法在集合中搜索项将是 O (1)。从 HashSet<T>检索项的唯一方法是枚举所有 O (n)的项。

我还没有找到任何解决这个问题的方法，除了制作我自己的 HashSet<T>或使用 Dictionary<K, V>。还有别的办法吗？

注:
我不想检查 HashSet<T>是否包含该项目。我希望获得对存储在 HashSet<T>中的项的引用，因为我需要更新它(而不需要用另一个实例替换它)。我传递给 TryGetItem的条目是相等的(根据我传递给构造函数的比较机制) ，但它不是相同的引用。

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HashSet has a Contains(T) method.

You can specify an IEqualityComparer if you need a custom comparison method (e.g., store a person object, but use the SSN for equality comparison).

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SortedSet would probably have O(log n) lookup time in that circumstance, if using that is an option. Still not O(1), but at least better.

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This is actually a huge omission in the set of collections. You would need either a Dictionary of keys only or a HashSet that allows for the retrieval of object references. So many people have asked for it, why it doesn't get fixed is beyond me.

Without third-party libraries the best workaround is to use Dictionary<T, T> with keys identical to values, since Dictionary stores its entries as a hash table. Performance-wise it is the same as the HashSet, but it wastes memory of course (size of a pointer per entry).

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
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You can also use ToList() method and apply an indexer to that.

HashSet<string> mySet = new HashSet();
mySet.Add("mykey");
string key = mySet.toList()[0];
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What about overloading the string equality comparer:

  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());
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Ok, 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. In order to update your object, then you should use:

yourHashSet.Where(w => w.Name.Contains("strin")).FirstOrDefault().MyProperty = "something";
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Modified implementation of @mp666 answer so it can be used for any type of HashSet and allows for overriding the default equality comparer.

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;
}
}
小开

Another Trick would do Reflection, by accessing the internal function InternalIndexOf of HashSet. Keep in mind the fieldnames are hardcoded, so if those change in upcoming .NET versions this will break.

Note: If you use Mono, you should change field name from m_slots to _slots.

internal static class HashSetExtensions<T>
{
public delegate bool GetValue(HashSet<T> source, T equalValue, out T actualValue);


public static GetValue TryGetValue { get; }


static HashSetExtensions() {
var targetExp = Expression.Parameter(typeof(HashSet<T>), "target");
var itemExp   = Expression.Parameter(typeof(T), "item");
var actualValueExp = Expression.Parameter(typeof(T).MakeByRefType(), "actualValueExp");


var indexVar = Expression.Variable(typeof(int), "index");
// ReSharper disable once AssignNullToNotNullAttribute
var indexExp = Expression.Call(targetExp, typeof(HashSet<T>).GetMethod("InternalIndexOf", BindingFlags.NonPublic | BindingFlags.Instance), itemExp);


var truePart = Expression.Block(
Expression.Assign(
actualValueExp, Expression.Field(
Expression.ArrayAccess(
// ReSharper disable once AssignNullToNotNullAttribute
Expression.Field(targetExp, typeof(HashSet<T>).GetField("m_slots", BindingFlags.NonPublic | BindingFlags.Instance)), indexVar),
"value")),
Expression.Constant(true));


var falsePart = Expression.Constant(false);


var block = Expression.Block(
new[] { indexVar },
Expression.Assign(indexVar, indexExp),
Expression.Condition(
Expression.GreaterThanOrEqual(indexVar, Expression.Constant(0)),
truePart,
falsePart));


TryGetValue = Expression.Lambda<GetValue>(block, targetExp, itemExp, actualValueExp).Compile();
}
}


public static class Extensions
{
public static bool TryGetValue2<T>(this HashSet<T> source, T equalValue,  out T actualValue) {
if (source.Count > 0) {
if (HashSetExtensions<T>.TryGetValue(source, equalValue, out actualValue)) {
return true;
}
}
actualValue = default;
return false;
}
}

Test:

var x = new HashSet<int> { 1, 2, 3 };
if (x.TryGetValue2(1, out var value)) {
Console.WriteLine(value);
}
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Now .NET Core 2.0 has this exact method.

HashSet.TryGetValue(T, T) Method

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This method has been added to .NET Framework 4.7.2 (and .NET Core 2.0 before it); see HashSet<T>.TryGetValue. Citing 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)
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最佳答案

What you're asking for was added to .NET Core a year ago, and was 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 above):

    //
// 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);

P.S.: In case you're interested, there is related function they're adding in the future - HashSet.GetOrAdd(T).


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