The hat (^) and range (..) operators in C#

The hat operator (^) and range operator (..) provide a different syntax for accessing elements in an array: Span, or ReadOnlySpan. The range operator is used to specify the start and end of a range for a sequence.

Let’s look at some examples. Consider the following code where you have an array of strings and you want to get the first four elements:

static void Main(string[] args)
{
  var people = new string[] { "Jane", "Jean", "Grey", "Marcus", "Theophilus", "Keje" };
  var firstFour = GetFirstFourPersons(people);

  foreach (var person in firstFour)
  {
    Console.WriteLine(person);
  }
}

static string[] GetFirstFourPersons(string[] people)
{
  var result = new string[4];
  for (int i = 0; i < 4; i++)
  {
    result[i] = people[i];
  }
  return result;
}

// result:
// Jane
// Jean
// Grey
// Marcus

We can rewrite it using the range operator, passing the range operand inside [ and ].

static void Main(string[] args)
{
  var people = new string[] { "Jane", "Jean", "Grey", "Marcus", "Theophilus", "Keje" };
  var firstFour = people[0..4];

  foreach (var person in firstFour)
  {
    Console.WriteLine(person);
  }
}

This gives the same result as the previous example. Before and after the .. operator are the start index and end index from which to get a sequence of data. It doesn't include the element in the end index as part of the result.

The range can also be open-ended. That means you can omit the start index, end index or both.

var all = people[..]; // contains all the elements from the origin array
var firstFour = people[..4]; // contains "Jane", "Jean", "Grey", and "Marcus"
var lastTwo = people[4..]; // contains "Theophilus" and "Keje"

The all variable is open-ended on both ends and therefore returns all the elements. It can also be written as var all = people[0..people.Length]. If you omit the start index, it'll use 0 as the start index, and if it's the end index, it'll use the value sequence.Length to resolve the value.

You can also declare range variables:

Range firstFourRange = ..4
var firstFour = people[firstFourRange]; // contains "Jane", "Jean", "Grey", and "Marcus"

With C# 8, you can specify that an index is relative to the end of the array. You do this using the ^ operator. Given the array people, we can get the last element in the sequence using people[^1]. The ^0 is the same as people.Length. So if you use people[^0] to get the last element of the sequence, you'll get an exception because it's outside the allowed range. We can use this with the range operator:

Range lastTwoElement = ^2..
var lastTwo = people[lastTwoElement] // contains "Theophilus" and "Keje"

This will give us the last two names. Omitting the end index translates to using ^0 (i.e. people.Length) as the end index. We can also assign the index to a variable:

Index lastIndex = ^1;
string value = people[lastIndex];

This language support is based on two new types, System.Index and System.Range. The Index type represents an index into a sequence, and the Range type specifies a sub-range of a sequence.

Source: Medium by MBARK T3STO

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