How is the C# Dictionary used this way in C# in depth 2nd Edition?

Question

I have come across the following code in C# in Depth 2nd Edition by Jon Skeet and I don't understand how it works.

Dictionary<string,int> frequencies;
frequencies = new Dictionary<string,int>();
string[] words = Regex.Split(text, @"\W+");
foreach (string word in words)
{
    if (frequencies.ContainsKey(word))
    {
        frequencies[word]++;
    }
    else
    {
        frequencies[word] = 1;
    }
}

Specifically how does the "word" key get added to the dictionary? As I see it, a new dictionary is created called frequencies, it is empty. There is then a method to split a string called text into an array of string using Regex.Split. So far all good. Next there is a foreach loop which loops through the array, but the next part trips me up, it is checking if frequencies contains the particular word, if it does then increase the value of it by 1 or if it doesn't yet have a value set it to 1. But how does the dictionary get populated with the "word" key in the first place to allow it to be checked?

It looks to happen in this line

frequencies[word] = 1;

But I can't find a reference anywhere that says specifying a dictionary object followed by square brackets and an assignment to a value also populates the key. I thought you needed to use the add method of the dictionary instance or do so when initializing the dictionary.

If I am correct what is the name of this action?

Answer 1

frequencies[word] = 1;

is the same as calling

frequencies.Add(word, 1);

if the key word does not already exist. Otherwise you override the value.

When you call [something] on a dictionary you get a value by key something. The same goes for setting. When setting a value you can call dictionary[key] = value.

The function used is the [] operator (brackets operator).

I dove into the Object Browser and found this about the [] operator of the generic dictionary:

public TValue this[TKey key] { get; set; } Member of System.Collections.Generic.Dictionary<TKey, TValue>

Summary: Gets or sets the value associated with the specified key.

Parameters: key: The key of the value to get or set.

Return Values: The value associated with the specified key. If the specified key is not found, a get operation throws a System.Collections.Generic.KeyNotFoundException, and a set operation creates a new element with the specified key.

Exceptions: System.ArgumentNullException: key is null. System.Collections.Generic.KeyNotFoundException: The property is retrieved and key does not exist in the collection.