C# ContainsKey Method (Key Exists in Dictionary)

Use the ContainsKey method on the Dictionary to test for key existence.


This is a Dictionary method. It computes the hash code for its argument. It then checks the internal structures in the Dictionary to see if that key exists.

Method notes.

ContainsKey is extremely fast. It does no linear searching. It uses a hash code to estimate where the data is stored.GetHashCode

An example.

You will find the ContainsKey method on a Dictionary. In Visual Studio, try typing the variable name and pressing period, and then scrolling to ContainsKey.Dictionary

Info: The program populates the Dictionary with 2 key-value pairs. ContainsKey is invoked twice.

Argument: The ContainsKey method receives one parameter. This is the key we wish to check for.

Returns: ContainsKey returns a Boolean value that indicates whether the key was found in the Dictionary or not.


And: You can test ContainsKey in an if-statement. It will not throw exceptions on a key that is not found.

IfTrue, False
C# program that uses ContainsKey using System; using System.Collections.Generic; class Program { static void Main() { // Create Dictionary with two key value pairs. var dictionary = new Dictionary<string, int>() { {"mac", 1000}, {"windows", 500} }; // Use ContainsKey method. if (dictionary.ContainsKey("mac") == true) { Console.WriteLine(dictionary["mac"]); // <-- Is executed } // Use ContainsKey method on another string. if (dictionary.ContainsKey("acorn")) { Console.WriteLine(false); // <-- Not hit } } } Output 1000


The .NET Framework implements the ContainsKey method. Dictionary has a FindEntry() method which loops over the entries in the Dictionary that are pointed to by the buckets array.

And: When it finds a matching hash code, it compares the key values and then returns the actual value.

Returns: The ContainsKey method discards some of the values it finds and simply returns a Boolean.

Therefore: Using TryGetValue can be used to perform these operations at one time, improving runtime speed.



A benchmark of ContainsKey compared to ContainsValue shows that ContainsKey is many times faster (the exact amount depends on the Dictionary size). This is important to know.ContainsValue

A summary.

ContainsKey checks for a key's existence. Dictionary implements a lightning-fast lookup table. But it will throw exceptions if you try to access an entry when no key exists.
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