Part 72 - What is dictionary in c#

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In thie video, we will discuss dictionary object in c#.
1. A dictionary is a collection of (key, value) pairs.
2. Dictionary class is present in System.Collections.Generic namespace.
3. When creating a dictionary, we need to specify the type for key and value.
4. Dictionary provides fast lookups for values using keys.
5. Keys in the dictionary must be unique.



Here is an example. The code is commented and is self-explanatory.
public class Program
{
    public static void Main()
    {
        // Create a Dictionary, CustomerID is the key. Type is int
        // Customer object is the value. Type is Customer
        Dictionary<int, Customer> dictionaryCustomers = new Dictionary<int, Customer>();

        // Create Customer Objects
        Customer customr1 = new Customer()
        {
            ID = 101,
            Name = "Mark",
            Salary = 5000
        };

        Customer customr2 = new Customer()
        {
            ID = 102,
            Name = "Pam",
            Salary = 7000
        };

        Customer customr3 = new Customer()
        {
            ID = 104,
            Name = "Rob",
            Salary = 5500
        };

        // Add customer objects to the dictionary
        dictionaryCustomers.Add(customr1.ID, customr1);
        dictionaryCustomers.Add(customr2.ID, customr2);
        dictionaryCustomers.Add(customr3.ID, customr3);

        // Retrieve the value (Customer object) from the dictionary,
        // using key (customer ID). The fastest way to get a value
        // from the dictionary is by using its key
        Console.WriteLine("Customer 101 in customer dictionary");
        Customer customer101 = dictionaryCustomers[101];
        Console WriteLine("ID = {0}, Name = {1}, Salary = {2}",
                            customer101.ID, customer101.Name, customer101.Salary);
        Console.WriteLine("--------------------------------------------------");

        // It is also possible to loop thru each key/value pair in a dictionary
        Console.WriteLine("All customer keys and values in customer dictionary");
        foreach (KeyValuePair<int, Customer> customerKeyValuePair in dictionaryCustomers)
        {
            Console.WriteLine("Key = " + customerKeyValuePair.Key);
            Customer cust = customerKeyValuePair.Value;
            Console.WriteLine("ID = {0}, Name = {1}, Salary = {2}", cust.ID, cust.Name, cust.Salary);
        }
        Console.WriteLine("--------------------------------------------------");

        // You can also use implicitly typed variable VAR to 
        // loop thru each key/value pair in a dictionary. But try
        // to avoid using var, as this makes your code less readable
        Console.WriteLine("All customer keys and values in customer dictionary");
        foreach (var customerKeyValuePair in dictionaryCustomers)
        {
            Console.WriteLine("Key = " + customerKeyValuePair.Key);
            Customer cust = customerKeyValuePair.Value;
            Console.WriteLine("ID = {0}, Name = {1}, Salary = {2}", cust.ID, cust.Name, cust.Salary);
        }
        Console.WriteLine("--------------------------------------------------");

        // To get all the keys in the dictionary
        Console.WriteLine("All Keys in Customer Dictionary");
        foreach (int key in dictionaryCustomers.Keys)
        {
            Console.WriteLine(key);
        }
        Console.WriteLine("--------------------------------------------------");

        // To get all the values in the dictionary
        Console.WriteLine("All Customer objects in Customer Dictionary");
        foreach (Customer customer in dictionaryCustomers.Values)
        {
            Console.WriteLine("ID = {0}, Name = {1}, Salary = {2}", customer.ID, customer.Name, customer.Salary);
        }

        // If you try to add a key that already exists in the dictionary you 
        // will get an exception - An item with same key has already been 
        // added. So, check if the key already exists
        if (!dictionaryCustomers.ContainsKey(101))
        {
            dictionaryCustomers.Add(101, customr1);
        }

        // When accessing a dictionary value by key, make sure the dictionary 
        // contains the key, otherwise you will get KeyNotFound exception.
        if (dictionaryCustomers.ContainsKey(110))
        {
            Customer cus = dictionaryCustomers[110];
        }
        else
        {
            Console.WriteLine("Key does not exist in the dictionary");
        }
    }
}

public class Customer
{
    public int ID { get; set; }
    public string Name { get; set; }
    public int Salary { get; set; }
}