KeyValuePair

This C# type joins two things together—for example, a string can be associated with an int or another string. We loop over these pairs when using a Dictionary.

As a generic struct type in the C# language, KeyValuePair must have both its key and value types specified during creation time. The syntax can become a bit difficult.

Example

Here we use the KeyValuePair struct in a List. We store pairs of values in a single List—two Lists could be used, but that might complicate matters.

using System;
using System.Collections.Generic;

// Part 1: create a List of KeyValuePairs.
var list = new List<KeyValuePair<string, int>>();
list.Add(new KeyValuePair<string, int>("Cat", 1));
list.Add(new KeyValuePair<string, int>("Dog", 2));
list.Add(new KeyValuePair<string, int>("Rabbit", 4));

// Part 2: loop over list and print pairs.
foreach (var element in list)
{
    Console.WriteLine(element);
}
[Cat, 1]
[Dog, 2]
[Rabbit, 4]

Return, KeyValuePair

Often we need to return 2 values from a method. We can do this with KeyValuePair. Here is an example of the syntax—it can be tricky at first to use.

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        var result = GetFileType();
        Console.WriteLine("TYPE: {0}, EXTENSION: {1}", result.Key, result.Value);
    }

    static KeyValuePair<string, string> GetFileType()
    {
        // Gets file type information.
        string type = "image/avif";
        string extension = "avif";
        return new KeyValuePair<string, string>(type, extension);
    }
}
TYPE: image/avif, EXTENSION: avif

Foreach, Dictionary

A common use of KeyValuePair is in a loop over a Dictionary. The Dictionary has an enumerator that returns each key and value in a KeyValuePair, one at a time.

using System;
using System.Collections.Generic;

var animals = new Dictionary<string, int>()
{
    { "cat", 2 }, { "dog", 1 }
};

// Use KeyValuePair with foreach on Dictionary.
foreach (KeyValuePair<string, int> animal in animals)
{
    Console.WriteLine(animal);
}
[cat, 2]
[dog, 1]

Sort, lambda

To sort KeyValuePairs, we can use Comparison methods. A lambda is the cleanest way to do this. Here we sort in descending order by the Value of each pair.

using System.Collections.Generic;

var data = new List<KeyValuePair<int, int>>()
{
    new KeyValuePair<int, int>(1, 6),
    new KeyValuePair<int, int>(1, 2),
    new KeyValuePair<int, int>(3, 4)
};
// Sort pairs in list in descending order based on the value.
// ... Use reverse order of A and B to mean descending sort.
data.Sort((a, b) => (b.Value.CompareTo(a.Value)));
foreach (var pair in data)
{
    System.Console.WriteLine(pair);
}
[1, 6]
[3, 4]
[1, 2]

ToString

When we want to display the values, call ToString or pass the KeyValuePair to Console.Write or Console.WriteLine. This will implicitly call ToString.

using System;
using System.Collections.Generic;

var pair = new KeyValuePair<string, int>("bird", 10);
// Get string from KeyValuePair.
string result = pair.ToString();
Console.WriteLine("TOSTRING: {0}", result);
Console.WriteLine("UPPER:    {0}", result.ToUpper());
TOSTRING: [bird, 10]
UPPER:    [BIRD, 10]

Read-only error

When using KeyValuePair, we may get this error. The C# compiler doesn't allow us to assign the Key and Value properties. They must be assigned in the constructor.

using System.Collections.Generic;

class Program
{
    static void Main()
    {
        var pair = new KeyValuePair<string, int>("bird", 10);
        pair.Key = "rat";
    }
}
Property or indexer 'System.Collections.Generic.KeyValuePair...Key'
cannot be assigned to--it is read-only.

Implementation

Here is the basic layout of the KeyValuePair struct. The KeyValuePair has 2 private fields, and 2 public properties that retrieve the values of those fields.

[Serializable, StructLayout(LayoutKind.Sequential)]
public struct KeyValuePair<TKey, TValue>
{
    private TKey key;
    private TValue value;
    public KeyValuePair(TKey key, TValue value);
    public TKey Key { get; }
    public TValue Value { get; }
    public override string ToString();
}

Benchmark, memory use

KeyValuePair wastes no memory—it is compact. Consider this example program: it has 2 versions of code, each of which store keys and values.

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        const int size = 10000;

        Console.WriteLine("::2 ARRAYS::");
        TestArrays(size);

        Console.WriteLine("::1 KEYVALUEPAIR ARRAY::");
        TestKeyValuePairs(size);
    }

    static void TestArrays(int size)
    {
        // Version 1: allocate 2 arrays.
        long mem1 = GC.GetTotalMemory(false);

        string[] keys = new string[size];
        int[] values = new int[size];
        keys[0] = "bird";
        values[1] = 10;

        long mem2 = GC.GetTotalMemory(false);

        // Collect garbage.
        GC.Collect();
        Console.WriteLine(mem2 - mem1);
        keys[0] = "";
    }

    static void TestKeyValuePairs(int size)
    {
        // Version 2: allocate 1 array of KeyValuePairs.
        long mem1 = GC.GetTotalMemory(false);

        KeyValuePair<string, int>[] array = new KeyValuePair<string, int>[size];
        array[0] = new KeyValuePair<string, int>("bird", 10);

        long mem2 = GC.GetTotalMemory(false);

        // Collect garbage.
        GC.Collect();
        Console.WriteLine(mem2 - mem1);
        array[0] = new KeyValuePair<string, int>("", 0);
    }
}
::2 ARRAYS::
80048
::1 KEYVALUEPAIR ARRAY::
80024

Benchmark, structs

Is there any advantage to using custom structs instead of KeyValuePair generic types? The 2 approaches are equivalent in functionality.

using System;
using System.Collections.Generic;
using System.Diagnostics;

struct CustomPair
{
    public int Key;
    public string Value;
}

class Program
{
    const int _max = 300000000;
    static void Main()
    {
        CustomPair p1;
        p1.Key = 4;
        p1.Value = "perls";
        Method(p1);

        KeyValuePair<int, string> p2 = new KeyValuePair<int, string>(4, "perls");
        Method(p2);

        for (int a = 0; a < 5; a++)
        {
            var s1 = Stopwatch.StartNew();
            // Version 1: use custom struct.
            for (int i = 0; i < _max; i++)
            {
                Method(p1);
                Method(p1);
            }
            s1.Stop();
            var s2 = Stopwatch.StartNew();
            // Version 2: use KeyValuePair.
            for (int i = 0; i < _max; i++)
            {
                Method(p2);
                Method(p2);
            }
            s2.Stop();

            Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
            Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
        }
    }

    static int Method(CustomPair pair)
    {
        return pair.Key + pair.Value.Length;
    }

    static int Method(KeyValuePair<int, string> pair)
    {
        return pair.Key + pair.Value.Length;
    }
}
0.32 ns    CustomPair
4.35 ns    KeyValuePair
0.32 ns
4.34 ns
0.32 ns
4.36 ns
0.32 ns
4.35 ns
0.32 ns
4.36 ns

If you think carefully, keys and values are everywhere—a term has a definition, and an action has a result. It makes sense that KeyValuePair has many uses throughout C# programs.