ToDictionary Method

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In C#, the ToDictionary method is a LINQ extension method that allows you to create a dictionary from a collection. The ToDictionary method is a useful tool for transforming a collection into a dictionary, where each item in the collection is used as a key in the dictionary. In this lesson, we will explore the ToDictionary method in detail, including its definition, syntax, and examples.

Definition:

The ToDictionary method is a LINQ extension method that is used to create a dictionary from a collection. The ToDictionary method works by iterating through each item in the collection, using a specified key selector to determine the key for each item, and adding the key-value pair to the dictionary. If the key selector produces duplicate keys, an ArgumentException is thrown.

Syntax:

The syntax for the ToDictionary method is as follows:

Dictionary<TKey, TElement> ToDictionary<TSource, TKey, TElement>(
    this IEnumerable<TSource> source,
    Func<TSource, TKey> keySelector,
    Func<TSource, TElement> elementSelector
)

The ToDictionary method takes three parameters: the source collection, a key selector function that selects the key for each item in the collection, and an element selector function that selects the value for each item in the collection. The generic type parameters, TKey and TElement, specify the type of the key and value in the dictionary.

Example 1: Creating a Dictionary from a List of Objects

Let’s say we have a List of objects that contain a name and an age, and we want to create a dictionary where the name is the key and the age is the value. We can use the ToDictionary method to accomplish this as follows:

List<Person> people = new List<Person>()
{
    new Person { Name = "John", Age = 30 },
    new Person { Name = "Jane", Age = 25 },
    new Person { Name = "Bob", Age = 40 }
};

Dictionary<string, int> ageDictionary = people.ToDictionary(p => p.Name, p => p.Age);

In this example, we first create a List of Person objects called people. We then use the ToDictionary method to create a dictionary where the name of each person is the key and the age of each person is the value. The key selector function selects the Name property of each Person object, and the element selector function selects the Age property of each Person object. The result is a Dictionary called ageDictionary.

Example 2: Creating a Dictionary with a Custom Key Selector

Let’s say we have a List of integers, and we want to create a dictionary where the keys are the squares of the integers. We can use the ToDictionary method with a custom key selector to accomplish this as follows:

List<int> numbers = new List<int>() { 1, 2, 3, 4, 5 };

Dictionary<int, int> squareDictionary = numbers.ToDictionary(n => n * n);

In this example, we first create a List of integers called numbers. We then use the ToDictionary method with a custom key selector that selects the square of each number as the key. The result is a Dictionary called squareDictionary, where the keys are the squares of the integers.

Conclusion:

The ToDictionary method is a useful tool for transforming a collection into a dictionary in C#. By understanding how to use the ToDictionary method and its syntax, developers can create dictionaries from collections with ease and customize the key and value selectors to fit their specific needs. The ToDictionary method can simplify code and improve performance, especially when working with large collections.