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C# Dictionary

A Dictionary is a collection that pairs each key with one value. A key is looked up to read its value.

Add and Read Pairs

The indexer adds two pairs of name and age. One age prints, then the pair count.

using System;
using System.Collections.Generic;

class Program
{
    static void Main(string[] args)
    {
        Dictionary<string, int> ages = new Dictionary<string, int>();
        ages["Hana"] = 27;
        ages["Bo"] = 31;
        Console.WriteLine(ages["Hana"]);
        Console.WriteLine(ages.Count);
    }
}

Update and Check Keys

Assigning to the key Hana replaces the old age. ContainsKey answers for a present key and a missing key.

using System;
using System.Collections.Generic;

class Program
{
    static void Main(string[] args)
    {
        Dictionary<string, int> ages = new Dictionary<string, int>();
        ages["Hana"] = 27;
        ages["Bo"] = 31;
        ages["Hana"] = 28;
        Console.WriteLine(ages["Hana"]);
        Console.WriteLine(ages.ContainsKey("Bo"));
        Console.WriteLine(ages.ContainsKey("Mina"));
    }
}

List Pairs in Order

The keys are copied into a list and sorted first. The loop then prints each pair in key order.

using System;
using System.Collections.Generic;

class Program
{
    static void Main(string[] args)
    {
        Dictionary<string, int> ages = new Dictionary<string, int>();
        ages["Hana"] = 27;
        ages["Bo"] = 31;
        List<string> keys = new List<string>(ages.Keys);
        keys.Sort();
        foreach (string key in keys)
        {
            Console.WriteLine(key + ": " + ages[key]);
        }
    }
}

Example

using System;
using System.Collections.Generic;

class Program
{
    static void Main(string[] args)
    {
        Dictionary<string, int> stock = new Dictionary<string, int>();
        stock["Pens"] = 8;
        stock["Pencils"] = 12;
        stock["Rulers"] = 5;
        List<string> keys = new List<string>(stock.Keys);
        keys.Sort();
        foreach (string key in keys)
        {
            Console.WriteLine(key + ": " + stock[key]);
        }
        Console.WriteLine("Kinds: " + stock.Count);
    }
}

Key points

  • Dictionary<K, V>: a collection that pairs keys with values.
  • The first type in the brackets is the key type.
  • The second type in the brackets is the value type.
  • Write a pair by assigning to the key in square brackets.
  • Read a value by writing its key in square brackets.
  • Count: a property that holds how many pairs the dictionary holds.
  • ContainsKey reports whether a key is present.

In this example, the dictionary holds a stock count for each item name. The keys are copied into a list and sorted. The loop prints Pencils with 12, then Pens with 8, then Rulers with 5. The last line prints the Count of pairs, which is 3.

Note

A key can appear only once. Assigning to it again updates the pair instead of adding one.

Try it Yourself

Edit the code, then press Run.

Running is simulated

Exercise

What does ages["Hana"] = 28 do when the key "Hana" already holds 27?