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Hashtable and Dictionary Collection Types 

The Hashtable class and the Dictionary generic class implement the IDictionary interface. The Dictionary generic class also implements the IDictionary generic interface. Therefore, each element in these collections is a key-and-value pair.

A Hashtable object consists of buckets that contain the elements of the collection. A bucket is a virtual subgroup of elements within the Hashtable, which makes searching and retrieving easier and faster than in most collections. Each bucket is associated with a hash code, generated using a hash function and based on the key of the element.

A hash function is an algorithm that returns a numeric hash code based on a key. The key is the value of some property of the object being stored. A hash function must always return the same hash code for the same key. It is possible for a hash function to generate the same hash code for two different keys, but a hash function that generates a unique hash code for each unique key results in better performance when retrieving elements from the hash table.

Each object that is used as an element in a Hashtable must be able to generate a hash code for itself using an implementation of the GetHashCode method. However, you can also specify a hash function for all elements in a Hashtable by using a Hashtable constructor that accepts an IHashCodeProvider implementation as one of its parameters.

When an object is added to a Hashtable, it is stored in the bucket that is associated with the hash code that matches the object's hash code. When a value is being searched for in the Hashtable, the hash code is generated for that value, and the bucket associated with that hash code is searched.

For example, a hash function for a string might take the ASCII codes of each character in the string and add them together to generate a hash code. The string "picnic" would have a hash code that is different from the hash code for the string "basket"; therefore, the strings "picnic" and "basket" would be in different buckets. In contrast, "stressed" and "desserts" would have the same hash code and would be in the same bucket.

The Dictionary class has the same functionality as the Hashtable class. A Dictionary of a specific type (other than Object) has better performance than a Hashtable for value types because the elements of Hashtable are of type Object and, therefore, boxing and unboxing typically occur if storing or retrieving a value type.

See Also