Get all Dictionary keys having some value in C#

This post will discuss how to get all Dictionary keys having some value in C#.

1. Using `Where()` method

We can use the Where() method to filter a dictionary based on a predicate. The following code example demonstrates how we can use Where() with Select() method to fetch all the dictionary keys having a value of 2. Note that it requires LINQ and you need to add System.Linq namespace.

using System;

using System.Linq;

using System.Collections.Generic;

public class Example

{

public static void Main()

{

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

{"A", 1}, {"B", 2}, {"C", 3}, {"D", 4}, {"E", 2}

};

var matchingKeys = dict.Where(kvp => kvp.Value == 2)

.Select(kvp => kvp.Key);

Console.WriteLine(String.Join(", ", matchingKeys)); // B, E

}

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2. Using `Dictionary<TKey,TValue>.FirstOrDefault()` Method

If all the values in the dictionary are unique or you need the first matching key, you can use the Dictionary<TKey,TValue>.FirstOrDefault() method. It returns the first element of a dictionary that satisfies a condition, or a default value if no element is found. This also requires LINQ.

using System;

using System.Linq;

using System.Collections.Generic;

public class Example

{

public static void Main()

{

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

{"A", 1}, {"B", 2}, {"C", 3}, {"D", 4}, {"E", 2}

};

var key = dict.FirstOrDefault(x => x.Value == 2).Key;

Console.WriteLine(key); // B

}

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3. Using `foreach` loop

Alternatively, you can write your custom logic for getting all Dictionary keys having specific value. Here’s what the code would look like using a foreach loop:

using System;

using System.Collections.Generic;

public static class Extensions

{

public static List<T> GetKeysByValue<T, W>(this IDictionary<T, W> dict, W value)

{

List<T> keys = new List<T>();

foreach (KeyValuePair<T, W> kvp in dict)

{

if (EqualityComparer<W>.Default.Equals(kvp.Value, value)) {

keys.Add(kvp.Key);

}

return keys;

}

public class Example

{

public static void Main()

{

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

{"A", 1}, {"B", 2}, {"C", 3}, {"D", 4}, {"E", 2}

};

var keys = dict.GetKeysByValue(2);

Console.WriteLine(String.Join(", ", keys)); // B, E

}

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That’s all about getting all Dictionary keys having some value in C#.

Get all Dictionary keys having some value in C#

1. Using Where() method

2. Using Dictionary<TKey,TValue>.FirstOrDefault() Method

3. Using foreach loop

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1. Using `Where()` method

2. Using `Dictionary<TKey,TValue>.FirstOrDefault()` Method

3. Using `foreach` loop