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C# ContainsKey Method

This C# example program uses the ContainsKey method on the Dictionary. ContainsKey tests for key existence.

ContainsKey is a Dictionary method.

It computes the hashcode for its argument. It then checks the internal structures in the Dictionary to see if that key exists. It is extremely fast. It does no linear searching.


Example. First, you will find the ContainsKey method on the Dictionary instance in your program by typing the variable name and pressing period, and then scrolling to ContainsKey. This method receives one parameter.

ContainsKey returns a Boolean value that indicates whether the key was found in the Dictionary or not. It will not throw if there were no keys matching. This example uses the ContainsKey method twice.


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


1000    Evaluation result for dictionary["mac"]

The program defines the Main entry point and then populates the Dictionary with two key-value pairs. The object creation expression at the start of Main is actually compiled into two Add method invocations.

Next: ContainsKey is invoked twice. You can test ContainsKey in an if-statement by comparing it to true or false.

IfTrue, False

Internals. The .NET Framework's base class library implements the ContainsKey method on the Dictionary type. The Dictionary has a private FindEntry method which loops over the entries in the Dictionary that are pointed to by the buckets array.

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


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 without using ContainsKey or TryGetValue initially.