Dictionary

Dictionary<TKey, TValue> is a key value pair collection, where you specify the type for both the key and the value.

to get started, let's create an oldschool console application with a main

dotnet new console -n pav.dictionaryExample --use-program-main

with our console application created open it in ms code

code ./pav.dictionaryExample 

and that should open your newly created dotnetcore project.

in our example let's create a person class,  we are going 

namespace DictionaryExample

{

    class Person

    {

        public string Name { get; set; }

        public DateTime BirthDate { get; set; }

        public Person(string Name, DateTime BirthDate) {

            this.Name = Name;

            this.BirthDate = BirthDate;

        }

        public int GetAge() {

            DateTime today = DateTime.Today;

            int age = today.Year - BirthDate.Year;

            return BirthDate > today.AddYears(-age) ? --age : age;

        }

        public override string ToString() { return $"{Name} is {GetAge()}"; }

    }

    class Program {

        static void Main(string[] args) {

            var d = new Dictionary<string, Person>();

            var p1 = new Person("Pavel", new DateTime(1984, 01, 31));

            var p2 = new Person("Tomek", new DateTime(1988, 08, 28));

            var p3 = new Person("Magda", new DateTime(1984, 06, 28));

            d.Add(p1.Name, p1);

            d.Add(p2.Name, p2);

            d.Add(p3.Name, p3);

            foreach (KeyValuePair<string, Person> de in d)

                Console.WriteLine("{0}: {1}", de.Key, de.Value.ToString());

            //print out a direct member of the dictionary by itS defined key.

            Console.WriteLine (d["Tomek"]);

        }

    }

}

if you run the above code you get the following:

if you want to remove and element by it's key you can simply call

d.Remove("Pavel");

if your collection doesn't contain this key nothing happens which is nice, if you want to retrieve an item using the key, it's just as simple.

            Console.WriteLine("get an object by a non-existent key");

            Console.WriteLine (d["marin"]);

Except for the exception that gets thrown if you query for a key that doesn't exist.

which is why it is always a good practice to first check is the key is contained in the collection

            Console.WriteLine("get an object by a non-existent key");

            if(d.ContainsKey("marin"))

                Console.WriteLine (d["marin"]);

             

The dictionary class also contains a containsValue method which works the same way, but obviously would have a performance impact. They key property is optimized for retrieving data.

            var p4 = new Person("Marin", new DateTime(1983, 11, 02));

            if (!d.ContainsValue(p4))

                d.Add(p4.Name, p4);

one caveat is that searching by value is not exactly efficient, search by key when you can.