C# ContainsValue MethodUse the ContainsValue method on the Dictionary type. Test for value existence.
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. The ContainsValue method receives 1 parameter of the type specified as the value type in the Dictionary. When you declare the Dictionary, you specify 2 type parameters.
Generic Class, Method
And The second type parameter, TValue, is the type used when calling ContainsValue.
New A new Dictionary containing keys and values of string type is created. It is populated with animal names and their default colors.
Variables The variables flag1, flag2 and flag3 are assigned the results of ContainsValue.
Returns This example shows how ContainsValue() searches all entries in the Dictionary for a match and returns a bool.
C# program that uses ContainsValue
using System; using System.Collections.Generic; class Program { static void Main() { // 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"); // See if colors exists. bool flag1 = animalColors.ContainsValue("black"); bool flag2 = animalColors.ContainsValue("clear"); bool flag3 = animalColors.ContainsValue("blue"); // 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.
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).
C# program that benchmarks ContainsKey, ContainsValue
using System; using System.Collections.Generic; using System.Diagnostics; class Program { const int _max = 10000; static void Main() { // 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.
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