Dictionary Dictionary ContainsValue Dictionary TryGetValue foreach GetHashCode if String StartsWith

`ContainsKey`

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

C# method notes

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

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.

Part 1 The program populates the Dictionary with 2 key-value pairs. Each has a string key, and an int value.

Part 2 You can test ContainsKey in an if-statement. It will not throw exceptions on a key that is not found.

Part 3 If the key is not found, the inner body of the if-statement is not entered—so false is not printed here.

using System;
using System.Collections.Generic;

// Part 1: create Dictionary with 2 key value pairs.
var dictionary = new Dictionary<string, int>()
{
    { "mac", 1000 },
    { "windows", 500 }
};

// Part 2: use ContainsKey method.
if (dictionary.ContainsKey("mac") == true)
{
    Console.WriteLine(dictionary["mac"]);
}

// Part 3: use ContainsKey on a key that does not exist.
if (dictionary.ContainsKey("acorn"))
{
    Console.WriteLine(false);
}
1000

Foreach `Keys`

Suppose we want to scan the keys of a Dictionary to match a certain condition (like strings that end in a number). We can use a foreach-loop over the Keys property.

Note This approach is much slower than ContainsKey, but if it has the correct result and is needed, it can be helpful.

Part 1 We create a Dictionary with 2 string keys and 2 bool values using an initializer.

Part 2 We use a foreach-loop over the Keys property, and call EndsWith on each key.

using System;
using System.Collections.Generic;

// Part 1: create Dictionary.
var ids = new Dictionary<string, bool>()
{
    { "X1", true },
    { "X2", true }
};

// Part 2: use a foreach-loop on keys for more advanced scanning logic.
foreach (var key in ids.Keys)
{
    if (key.EndsWith("2"))
    {
        Console.WriteLine($"KEY ENDS WITH 2: {key}");
    }
}
KEY ENDS WITH 2: X2

Internals

The .NET runtime 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.

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

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

Benchmark

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.

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.

ContainsKey