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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 · Programiz C# Compiler

Exercise

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