C# - Dictionary ContainsValue Method

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.

Dictionary ContainsKey

Dictionary TryGetValue

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.

Dictionary

Generic

Step 1 A new Dictionary containing keys and values of string type is created. It is populated with animal names and their default colors.

Step 2 The variables flag1, flag2 and flag3 are assigned the results of ContainsValue.

Step 3 This example shows how ContainsValue() searches all entries in the Dictionary for a match and returns a bool.

bool

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.

KeyValuePair

Benchmark

Version 1 This version of the code uses ContainsValue. It repeatedly calls ContainsValue with a key that exists in the Dictionary.

Version 2 Here we use ContainsKey instead. The key exists in the Dictionary, and we access it many times.

Result Checking for the existence of a key (with ContainsKey) is much faster than checking for a value (with ContainsValue).

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.

And ContainsValue checks the value of each element. For this reason, using ContainsValue is far slower in most cases than ContainsKey.

A summary. 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.