Filter a Dictionary in C#

This post will discuss how to filter a Dictionary<TKey,TValue> in C#.

Since a Dictionary<TKey,TValue> implements IEnumerable<KeyValuePair<Key,Value>>, we can use the Where() method to filter it. The Where() method filters a sequence of values based on a predicate and is available in the System.Linq namespace. The following code example demonstrates its usage.

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", 5}

};

Dictionary<string, int> filtered = dict.Where(x => x.Value % 2 == 0)

.ToDictionary(x => x.Key, x => x.Value);

Console.WriteLine(String.Join(", ", filtered)); // [B, 2], [D, 4]

}

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The above solution invokes the ToDictionary() method on the filtered sequence to get a new dictionary with associated key-value pairs. If you don’t want to create a new dictionary and need to modify the original dictionary, do like:

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", 5}

};

foreach (var item in dict.Where(x => x.Value % 2 == 0).ToList()) {

dict.Remove(item.Key);

}

Console.WriteLine(String.Join(", ", dict)); // [A, 1], [C, 3], [E, 5]

}

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Note that we’re modifying the underlying collection, ToList() is needed here. Otherwise, System.InvalidOperationException exception will be thrown at the next loop iteration, as shown below:

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", 5}

};

foreach (var item in dict.Where(x => x.Value % 2 == 0)) {

dict.Remove(item.Key);

}

Console.WriteLine(String.Join(", ", dict)); // [A, 1], [C, 3], [E, 5]

}

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Output:

Unhandled Exception:
System.InvalidOperationException: Collection was modified; enumeration operation may not execute.

That’s all about filtering a Dictionary<TKey,TValue> in C#.

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