How to get the value of the HashSet collection?

Your question is generally meaningless, since for a string the operation "get" value from a hashset is practically useless. Already have the string "JAPAN"? Do you want to get a special string "JAPAN" from the hashstat? I will disappoint you - it will be exactly the same as yours (and perhaps the same thanks to internment). Actually, what's the point of her "getting"?

As you yourself noted, in this class there is a Contains method, it indicates whether the required value is contained in a hashset, this is quite enough. If necessary, you can use a loop, or a wrapper like linq, if you do not want to use the loop (but most likely the loop will be activated inside). For example, something like this:

 string japan = country.FirstOrDefault(x => x == "JAPAN"); 

Meaning may have the receipt of values ​​for some other conditions other than equality. for example, all values ​​from 7 characters long:

 var greaterThanSeven = country.Where(x => x.Length > 7); 

Alternatively, getting values ​​can make sense for some complex data types, where filtering can be performed on individual fields. For example:

  var country = new HashSet<Foo> { new Foo { id = 1, name = "JAPAN" }, new Foo { id = 2, name = "USA" }, new Foo { id = 3, name = "AUSTRALIA" }, new Foo { id = 4, name = "CANADA" }, new Foo { id = 5, name = "CHINA" }, new Foo { id = 6, name = "RUSSIA" }, new Foo { id = 7, name = "FRANCE" }, }; var foo = country.FirstOrDefault(x => x.name == "JAPAN"); 

Good question. Only an unsuccessful example. Let's look at the class:

 class Class1 { private Int32 _value; public Int32 Value { get { return _value; } set { _value = value; } } public override int GetHashCode() { return _value.GetHashCode(); } public override bool Equals(object obj) { Class1 c1 = obj as Class1; if (c1 == null) return false; return c1._value == _value; } } 

Create a couple of objects and add them to the HashSet:

 Class1 c1 = new Class1() { Value = 1 }; Class1 c2 = new Class1() { Value = 2 }; HashSet<Class1> set = new HashSet<Class1>{c1, c2}; 

Next, we get the key by which we need to get an object from the HashSet:

 Class1 key1 = new Class1() { Value = 2 }; 

The bottom line is that the values ​​of c1 and key1 same, but they are two different objects:

 Console.WriteLine(set.Contains(key1)); //true Console.WriteLine(Object.ReferenceEquals(c2, key1)); // false 

And HashSet will show that it contains a similar object:

 Console.WriteLine(set.Contains(key1)); //true 

The object is similar, but not the same.

To get that object will have to do a search:

 Class1 result = set.FirstOrDefault(item => item.Equals(key1)); Console.WriteLine(Object.ReferenceEquals(c2, result)); // true 

It is impossible to get a specific value from HashSet.

If I understand correctly, you want to get the same element that you put with HashSet<T> . Specifically, with HashSet<T> for O (1), this is impossible, but it is easy to model with Dictionary<T, T> , if you place the same element as a key and value.

 T GetItemEqualTo(T pattern) { T result; if (dict.TryGetValue(pattern, out result)) return result; else return null; } 

You will need a generic constraint where T : class .

(This will not compile for the value-type T , but in this case the concept of the instance itself is meaningless.)

To get the value by key, you must use a Dictionary .

Different ones are needed precisely for checking availability, and possibly sorting. You can get any of these objects only in a foreach loop and in similar ways.