How to Create and Use the Dictionary in C#

A Dictionary in C# is a generic collection of key-value pairs. It is defined using the Dictionary<TKey, TValue> class, where TKey is the type of the keys and TValue is the type of the values. The Dictionary class is implemented as a hash table, which provides fast lookups and additions. It can be used to store and retrieve values based on their associated keys. For example:

Dictionary<string, int> grades = new Dictionary<string, int>();
grades.Add("sylvia",90);
grades.Add("sundar",70);
int johnsGrade = grades["rahul"];

Dictionary Characteristics

• Dictionary<TKey, TValue> stores key-value pairs.

• Comes under System.Collections.Generic namespace.

• Implements IDictionary<TKey, TValue> interface.

• Keys must be unique and cannot be null.

• Values can be null or duplicate.

• Values can be accessed the  bypassing associated key in the indexer e.g. yourDictionary[key]

• Elements are stored as KeyValuePair<TKey, TValue> objects.

Creating a Dictionary

Dictionary<string, int> dict = new Dictionary<string, int>();

Dictionary<string, int> dict = new Dictionary<string, int>
{
    {"John", 25},
    {"Jane", 30},
    {"Jim", 35}
};

Dictionary<string, int> dict = new Dictionary<string, int>();
dict.Add("John", 25);
dict.Add("Jane", 30);
dict.Add("Jim", 35);

C# dictionary initializer

A Dictionary Initializer in C# is a syntax that allows you to initialize a dictionary with values at the time of creation.

The syntax is as follows:

Dictionary<string, int> grades = new Dictionary<string, int>
{
    {"sylvia", 30},
    {"neupane", 70},
    {"mirmee", 25}

};

Each key-value pair is enclosed within curly braces {} and separated by a comma. The key is a string in this example, and the value is an integer.

This approach is convenient and readable, and it eliminates the need for multiple calls to the Add method to populate the dictionary.

C# dictionary foreach

You can use a foreach loop to iterate over the key-value pairs in a Dictionary in C#. The syntax is as follows:

Dictionary<string, int> dictionaryForeach= new Dictionary<string, int>
{
    {"sylvia", 60},
    {"neupane", 70},
    {"mirmee", 25}
};

foreach (KeyValuePair<string, int> item in dictionaryForeach)
{
    Console.WriteLine("Key: {0}, Value: {1}", item.Key, item.Value);
}

In each iteration, the KeyValuePair structure represents a single key-value pair in the dictionary. The Key property holds the key, and the Value property holds the value.

Alternatively, you can use the var keyword to simplify the syntax:

foreach (var item in dictionaryForeach)
{
    Console.WriteLine("Key: {0}, Value: {1}", item.Key, item.Value);
}

C# dictionary methods

C# dictionary add

Dictionary<string, int> dict = new Dictionary<string, int>();

dict.Add("Key1", 1);
dict.Add("Key2", 2);

bool success = dict.TryAdd("Key1", 3); // success = false

C# dictionary containskey

ContainsKey() Method. This method is used to check whether the Dictionary<TKey, TValue> contains the specified key or not. Syntax: public bool ContainsKey (TKey key);

To check if a specific key exists in a Dictionary in C#, you can use the ContainsKey method:

Dictionary<string, int> dict = new Dictionary<string, int>();
dict.Add("Key1", 1);
dict.Add("Key2", 2);
bool exists = dict.ContainsKey("Key1"); // exists = true

C# dictionary keys

You can create the Dictionary<TKey, TValue> object by passing the type of keys and values it can store. The following example shows how to create a dictionary to get a collection of all keys in a Dictionary in C#, you can use the Keys property:

Dictionary<string, int> dict = new Dictionary<string, int>();
dict.Add("Key1", 1);
dict.Add("Key2", 2);
var keys = dict.Keys;
foreach (var key in keys)
{
  Console.WriteLine(key);
}

C# dictionary get value

Dictionary<string, int> dict = new Dictionary<string, int>();
dict.Add("Key1", 1);
dict.Add("Key2", 2);

int value;
bool success = dict.TryGetValue("Key1", out value);
if (success)
{
    Console.WriteLine(value); // output: 1
}

int value = dict["Key1"];
Console.WriteLine(value); // output: 1

Dictionary<string, int> dictionary = new Dictionary<string, int>();

Dictionary<string, int> dictionary = new Dictionary<string, int>(10);

Dictionary<string, int> dictionary  = new Dictionary<string, int>(StringComparer.OrdinalIgnoreCase);

Dictionary<string, int> dictionary = new Dictionary<string, int>(10, StringComparer.OrdinalIgnoreCase);

IDictionary<string, int> sourceCode = new Dictionary<string, int>()
{
    { "Key1", 1 },
    { "Key2", 2 },
};

Dictionary<string, int> dictionary = new Dictionary<string, int>(sourceCode );

Dictionary<string, int> dicttionary = new Dictionary<string, int>()
{
    { "Key3", 3 },
    { "Key1", 1 },
    { "Key2", 2 },
};

var sortedList = dicttionary.OrderBy(x => x.Value).ToList();

foreach (var item in sortedList)
{
    Console.WriteLine($"{item.Key}: {item.Value}");
}

SortedDictionary<string, int> dicttionary = new SortedDictionary<string, int>()
{
    { "Key3", 3 },
    { "Key1", 1 },
    { "Key2", 2 },
};

foreach (var item in dicttionary)
{
    Console.WriteLine($"{item.Key}: {item.Value}");
}