C# LINQ Introduction

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Enumerables

We start with Enumerables. An enumerable in .Net is an object that allows you to enumerate a collection of elements.

Arrays

The most basic enumerable is an array, which can store a fixed number of typed elements.

int[] numbers = new int[3] { 1, 2, 3};

In the example above, we create an integer array that can hold 3 values and initialize the array with the values 1, 2 and 3. We assign that array to a variable of type int[] named numbers.

You can access each element of an array by index or enumerate the values.

// Access element by index
int fistNumber = numbers[0]; // 1
// Enumerate values
foreach(int number in numbers)
{
    Console.WriteLine(number)
}

/* Output:
 * 1
 * 2
 * 3
 */

Lists

Another common type of enumerables are lists. In contrast to arrays, lists have a variable size.

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

In the example above, we create a list of integers and initialize it with three values 1, 2 and 3. We assign it to a variable called numbers. In this example we use the keyword var to define the type of numbers. With var, we let the compiler decide which type to use. As we assign an object of type List<int> to numbers, the type of the variable will be List<int>.

Again, you can access the values by index, or enumerate them.

// Access element by index
var fistNumber = numbers[0]; // 1
// Enumerate values
foreach(var number in numbers)
{
    Console.WriteLine(number)
}

/* Output:
 * 1
 * 2
 * 3
 */

Sets

A third type of enumerables sets. Sets contain unique elements. The HashSet class for example, provides high-performance set operations (e.g. UnionWith, IntersectWith...).

var numbers = new HashSet<int>() {1, 2, 3};

In contrast to the previous two examples of arrays and lists, you cannot access an element of a set by it's index. There is no such concept of "index of an element" in a hash set.

However, you can still enumerate the values stored in the set.

// Enumerate values
foreach(var number in numbers)
{
    Console.WriteLine(number)
}

/* Output could be (but depends on the implementation of the set's enumerator):
 * 1
 * 2
 * 3
 */

And many more

There are many other types of enumerables in .Net, e.g. dictionaries, queues, stacks as well as other spezilized lists and sets.

The common ground

Although all enumerable types have different purposes and methods they have on common characteristic.

They all realize the same interface, namely IEnumerable<T>.

As enumerables are so fundamental in .Net, there is a declarative language construct in C# that makes working with enumerables easy.

We have seen it before: The foreach loop.

// Enumerate values
foreach(var element in objectOfTypeIEnumerable)
{
    // work with the element
}

One additional note: Generics types

The interface IEnumerable<T> is a generic interface. This means that the enumerable is strongly typed but you can define the concrete type in your code. Strongly typed means, that it can hold only elements of the specified type (or elements that can be implicitly converted to this type). T is the template type, a placehoder for the type you specify.

The type List<T> is also a generic type. The list was implemented to work with any type (<T>). However, you have to choose a fixed type in your code.

A List<int> (list of integers) for example is an IEnumerable<int> (enumerable of integers). You can add int values to the list. You can also add byte values to the list (as they can be converted to int automatically, i.e. implicitly, without loss). However, you can not add string or long values to the list. The former cannot be converted to an integer at all, the latter could be converted to an integer, but with a possible data loss and this is never done automatically.

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