ContainsValue

This C# method searches for a value in a Dictionary. Sometimes we cannot access a Dictionary only by looking up keys—we need to search for a specific value.

With this method, we can locate an entry by its value. ContainsValue is slow. It should not be used unless needed. It eliminates the performance benefits of a Dictionary.

First example

Here we call ContainsValue. When you declare the Dictionary, you specify 2 type parameters. The second type parameter, TValue, is the type used when calling ContainsValue.

using System;
using System.Collections.Generic;

// Step 1: store animal colors.
var animalColors = new Dictionary<string, string>();
animalColors.Add("bird", "blue");
animalColors.Add("worm", "pink");
animalColors.Add("cat", "black");
animalColors.Add("dog", "brown");

// Step 2: see if colors exists.
bool flag1 = animalColors.ContainsValue("black");
bool flag2 = animalColors.ContainsValue("clear");
bool flag3 = animalColors.ContainsValue("blue");

// Step 3: write results.
Console.WriteLine(flag1);
Console.WriteLine(flag2);
Console.WriteLine(flag3);
True
False
True

Benchmark

ContainsKey is many times faster than ContainsValue, and this is most apparent in large dictionaries. Consider this benchmark. A dictionary of 1000 key-value pairs is created.

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

const int _max = 10000;
// Create a lookup table.
var lookup = new Dictionary<string, string>();
for (int i = 0; i < 1000; i++)
{
    lookup[i.ToString() + "a"] = i.ToString() + "b";
}
// Version 1: use ContainsValue.
var s1 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    if (!lookup.ContainsValue("999b"))
    {
        return;
    }
}
s1.Stop();
// Version 2: use ContainsKey.
var s2 = Stopwatch.StartNew();
for (int i = 0; i < _max; i++)
{
    if (!lookup.ContainsKey("999a"))
    {
        return;
    }
}
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"));
13388.79 ns    ContainsValue
37.49 ns       ContainsKey

Discussion

ContainsKey() computes a hash code of the key and use that to locate the value for that key in near-constant time. ContainsValue() loops through all the entries.

With ContainsValue, a slow search of the Dictionary's values (not keys) is performed. This method should be avoided unless needed in a certain situation.