C# ContainsValue

ContainsValue 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. This method on the Dictionary type provides this functionality—but has a performance penalty.

Example

The ContainsValue method receives one parameter of the type specified as the value type in the Dictionary. When you declare the Dictionary, you specify two type parameters, and the second, TValue parameter is the type used in ContainsValue. This example shows how the ContainsValue searches all entries in the Dictionary for a match and returns a bool.

This C# example program uses the ContainsValue method on the Dictionary type.

Program that uses ContainsValue [C#]

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
	//
	// This dictionary contains stock symbols and market capatializations
	// Values are in billions of USD.
	//
	var caps = new Dictionary<string, decimal>();
	caps.Add("msft", 215);
	caps.Add("xon", 348);
	caps.Add("goog", 132);
	caps.Add("wmt", 188);

	//
	// See if there are stocks with these capitalizations.
	//
	bool flag1 = caps.ContainsValue(348);
	bool flag2 = caps.ContainsValue(1000);
	bool flag3 = caps.ContainsValue(188);

	//
	// The first call matches "xon";
	// The second call returns false;
	// The third call matches "wmt".
	//
	Console.WriteLine(flag1);
	Console.WriteLine(flag2);
	Console.WriteLine(flag3);
    }
}

Output

True
False
True

Overview. The program above defines a Program class that contains the Main entry point. A new Dictionary containing keys of type string and values of type decimal is instantiated and populated with four stock symbols and their market valuations in billions of USD. (The companies are Microsoft, Exxon, Google and Wal-Mart.)

Flags Boolean variables. The three bool variables flag1, flag2 and flag3 are assigned the results of ContainsValue on various parameters. Note that in Code Complete using the variable name of "flag" is discouraged because it does not describe the code's intent. In Refactoring, Martin Fowler suggests eliminating temporary variables and using the method calls themselves as the expressions.

ContainsValue versus ContainsKey

When you invoke ContainsKey, the method will compute a hash code of the key and use that to locate the value for that key in near-constant time. When you use ContainsValue, the method will loop through all the entries in the Dictionary and check the value of each element. For this reason, using ContainsValue is far slower in most cases than ContainsKey.

Summary

We saw how to use the ContainsValue method on the Dictionary generic class in the C# programming language targeting the .NET Framework. We saw an example of how this method can be used to search the values in a Dictionary. Additionally, we saw that it uses a linear, O(N) search, rendering it far slower than ContainsKey in most cases. It is similar to other searching methods such as Array.IndexOf or those that use simple for loops.