C# 0 The Complete Reference

PART II

C h a p t e r   2 4 :  

C o l l e c t i o n s ,   E n u m e r a t o r s ,   a n d   I t e r a t o r s    

797

HashSet

HashSet

 is a new collection added to the .NET Framework by version 3.5. It supports a 

collection that implements a set. It uses a hash table for storage. 

HashSet

implements

the

ICollection

,

IEnumerable

,

IEnumerable

,

ISerializable

, and 

IDeserializationCallback

 interfaces. 

HashSet

 implements a set in which all elements 

are unique. In other words, duplicates are not allowed. The order of the elements is not 

specified.

HashSet

 defines a full complement of set operations, such as intersection, 

union, and symmetric difference. This makes 

HashSet

 the perfect choice for working 

with sets of objects. 

HashSet

 is a dynamic collection that grows as needed to 

accommodate the elements it must store.

Here are four commonly used constructors defined by 

HashSet

:

public HashSet( )

public HashSet(IEnumerable 

c

)

public HashSet(IEqualityCompare 

comp

)

public HashSet(IEnumerable 

c

, IEqualityCompare 

comp

)

The first form creates an empty set. The second creates a set that contains the elements of 

the collection specified by 

c.

 The third lets you specify the comparer. The fourth creates a set 

that contains the elements in the collection specified by 

c

 and uses the comparer specified 

by

comp.

 There is also a fifth constructor that lets you initialize a set from serialized data.

In addition to the methods defined by the interfaces that it implements, 

HashSet

defines several of its own, most of which support various set operations. The set operation 

methods defined by 

HashSet

 are shown in Table 24-21. Notice that the arguments to 

these methods are 

IEnumerable

. This means you can pass something other than 

another

HashSet

 as the second set. Most often, however, both operands will be 

instances of 

HashSet

.

In addition to the properties defined by