Dictionary in C#

Dictionary in C# is a generic collection that stores key-value pairs. The working of Dictionary is quite similar to the non-generic hashtable. It's a generic type, which is defined under System.Collections.Generic namespace. It's dynamic in nature, meaning the size of the dictionary is growing as needed.

• The value is stored in the Key-Value pair.
• It provides fast lookups for values based on keys.
• Stores keys uniquely and adding duplicate keys results in a runtime exception.

Example 1: Creating and Displaying a Dictionary

Key: 1, Value: C#
Key: 2, Value: Javascript
Key: 3, Value: Dart

Steps to Create a Dictionary

Step 1: Include System.Collections.Generic namespace

using System.Collections.Generic;

Step 2: Create a Dictionary using Dictionary<TKey, TValue> class

Dictionary<TKey, TValue> dictionary_name = new Dictionary<TKey, TValue>();

Performing Different Operations on Dictionary

1. Adding Elements

• Add(): This method is used to add key/value pairs in your Dictionary.
• Collection-Initializer: We can also use a collection initializer to add elements to the dictionary.
• Using Indexers: We can directly add elements using indexers (keys, not indexes).

// Using Add method
Dictionary<int, string> dict= new Dictionary<int, string>();
dict.Add(1, "One");

// Using Collection Initializer
Dictionary<int, string> dict = new Dictionary<int, string>{
{ 1, "One" },
{ 2, "Two" }
};

// Using Indexers
Dictionary<int, string> dict = new Dictionary<int, string>();
dict[1] = "One";
dict[2] = "Two";

2. Accessing Elements

The key-value pair of the Dictionary is accessed using three different ways:

Using For Loop: We can use a for loop to access the key-value pairs of the Dictionary.

for (int x = 0; x < dict.Count; x++){
Console.WriteLine("{0} and {1}", dict.Keys.ElementAt(x), dict[dict.Keys.ElementAt(x)]);
}

Using Indexer: We can access individual key-value pairs of the Dictionary by using its key. We can specify the key in the index to get the value from the given dictionary, no need to specify the index. The indexer always takes the key as a parameter

Console.WriteLine("Value is:{0}", dict[1]);
Console.WriteLine("Value is:{0}", dict[2]);

Note: If the given key is not available in the dictionary, then it gives KeyNotFoundException.

Using foreach loop:

foreach (var pair in dict)
Console.WriteLine($"Key: {pair.Key}, Value: {pair.Value}");

Example: Creating and Displaying a Dictionary with Add()

key: 1 and value: Welcome
key: 2 and value: to
key: 3 and value: GeeksforGeeks

3. Removing Elements

In the Dictionary, we are allowed to remove elements from the Dictionary. Dictionary<TKey, TValue> class provides two different methods to remove elements that are:

• Clear(): This method removes all keys and values from the Dictionary<TKey, TValue>.
• Remove(): This method removes the value with the specified key from the Dictionary<TKey, TValue>.

Example:

key: 1, Value: Welcome
key: 2, Value: to
key: 3, Value: GeeksforGeeks

After Remove() method:
key: 2, Value: to
key: 3, Value: GeeksforGeeks

4. Checking Element

In Dictionary, we can check whether the given key or value is present in the specified dictionary or not. The Dictionary<TKey, TValue> class provides two different methods which are:

• ContainsKey: This method is used to check whether the Dictionary<TKey, TValue> contains the specified key.
• ContainsValue: This method is used to check whether the Dictionary<TKey, TValue> contains a specific value.

Example: