Csharp Dictionaries and Hashmaps Dictionary A dictionary is a collection of key-value pairs. It is a data structure that is used to store data in the form of key-value pairs. The key-value pair is also referred to as an entry. The key is used to retrieve the data associated with it. The key must be unique and immutable. The value can be changed. The value can be of any type. The key and value can be of the same type or of different types.