What is a Dictionary in C#?

A Dictionary in C# is a collection that stores key-value pairs, where each key is unique and used for quick access to the corresponding value. It implements the interfaces IDictionary<TKey, TValue>, ICollection<KeyValuePair<TKey, TValue>>, IEnumerable<KeyValuePair<TKey, TValue>>, IEnumerable. It is based on a hash table.

Main characteristics:

• Unordered: The order of elements in the dictionary is not guaranteed and may change when adding/removing items.
• Performance: Add, remove, and key-based search operations have an average complexity of O(1) (in an ideal case, without hash collisions) and O(n) in the worst case.
• Key uniqueness: Each key must be unique in the dictionary. Attempting to add an element with an existing key will result in an exception.

Example usage:

// Creating a dictionary
Dictionary<string, int> students = new Dictionary<string, int>();

// Adding elements
students.Add("Ivanov", 5);
students["Petrov"] = 4; // Alternative way to add/update

// Checking for a key
if (students.ContainsKey("Ivanov"))
{
    // Getting the value by key
    int mark = students["Ivanov"]; // mark == 5
}

// Iterating over elements
foreach (KeyValuePair<string, int> student in students)
{
    // student.Key - key, student.Value - value
    Console.WriteLine($"{student.Key}: {student.Value}");
}

// Removing an element
students.Remove("Petrov");

Comparison with other collections:

Collection	Main purpose	Index access	Key access	Key uniqueness	Ordered
List<T>	Sequence of elements	Yes	No	No	Yes
HashSet<T>	Unique set of elements	No	No	Yes	No
Dictionary<TKey, TValue>	Key-value pairs for quick search	No	Yes	Yes	No