What is Dictionary Generic Class?

Represents a collection of keys and values.
The Dictionary generic class provides a mapping from a set of keys to a set of values. Each addition to the dictionary consists of a value and its associated key. Retrieving a value by using its key is very fast, close to O(1), because the Dictionary class is implemented as a hash table.
The capacity of a Dictionary is the number of elements the Dictionary can hold. As elements are added to a Dictionary, the capacity is automatically increased as required by reallocating the internal array.