C# ContainsValue Method (Value Exists in Dictionary) - Dot Net Perls

Home Search	C# ContainsValue Method (Value Exists in Dictionary) Use the ContainsValue method on the Dictionary type. Test for value existence.
	ContainsValue. This searches for a value in a Dictionary. Sometimes you cannot access a Dictionary only by looking up keys and 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.Dictionary 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. 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); } } Output 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 Results: 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")); Console.Read(); } } Output 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. ContainsKey
	A summary. We saw how to use the ContainsValue method on the Dictionary. We saw an example of how this method can be used to search the values in a Dictionary.
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