Hash table


Each element is a key/value pair stored in a  DictionaryEntry object. A key cannot be null, but a value can be.

The objects used as keys by a Hashtable are required to override the  Object.GetHashCode method (or the  IHashCodeProvider interface) and the Object.Equals method (or the  IComparer interface). The implementation of both methods and interfaces must handle case sensitivity the same way; otherwise, the Hashtable might behave incorrectly. For example, when creating a Hashtable, you must use the  CaseInsensitiveHashCodeProvider class (or any case-insensitive  IHashCodeProvider implementation) with the  CaseInsensitiveComparer class (or any case-insensitive IComparer implementation).

Furthermore, these methods must produce the same results when called with the same parameters while the key exists in the Hashtable. An alternative is to use a Hashtable constructor with an  IEqualityComparer parameter. If key equality were simply reference equality, the inherited implementation of Object.GetHashCode and  Object.Equals would suffice.

Key objects must be immutable as long as they are used as keys in the Hashtable.

When an element is added to the Hashtable, the element is placed into a bucket based on the hash code of the key. Subsequent lookups of the key use the hash code of the key to search in only one particular bucket, thus substantially reducing the number of key comparisons required to find an element.

The load factor of a Hashtable determines the maximum ratio of elements to buckets. Smaller load factors cause faster average lookup times at the cost of increased memory consumption. The default load factor of 1.0 generally provides the best balance between speed and size. A different load factor can also be specified when the Hashtable is created.

As elements are added to a Hashtable, the actual load factor of the Hashtable increases. When the actual load factor reaches the specified load factor, the number of buckets in the Hashtable is automatically increased to the smallest prime number that is larger than twice the current number of Hashtablebuckets.

Each key object in the Hashtable must provide its own hash function, which can be accessed by calling  GetHash. However, any object implementing IHashCodeProvider can be passed to a Hashtable constructor, and that hash function is used for all objects in the table.

The capacity of a Hashtable is the number of elements the Hashtable can hold. As elements are added to a Hashtable, the capacity is automatically increased as required through reallocation.

For very large Hashtable objects, you can increase the maximum capacity to 2 billion elements on a 64-bit system by setting the enabled attribute of the gcAllowVeryLargeObjects configuration element to true in the run-time environment.

The foreach statement of the C# language (For Each in Visual Basic) returns an object of the type of the elements in the collection. Since each element of the Hashtable is a key/value pair, the element type is not the type of the key or the type of the value. Instead, the element type is DictionaryEntry.