C# 8 Indices and Ranges

Indices and ranges provide a succinct syntax for accessing single elements or ranges in a sequence. In C# 8.0, the following two new types are added.

• Index
• Range

You can use these structs to index or slice collections at runtime.

Index

The System.Index struct represents a type that can be used to index a collection either from the start or the end.

• It represents an index in an array or sequence.
• The ^ operator specifies the relative index from the end of an array.

Let's consider an array nameOfMonths which contains all the names of months.

string[] nameOfMonths = { 
                          // index from start    index from end
    "January",            // 0                   ^12
    "February",           // 1                   ^11
    "March",              // 2                   ^10
    "April",              // 3                   ^9
    "May",                // 4                   ^8
    "June",               // 5                   ^7
    "July",               // 6                   ^6
    "August",             // 7                   ^5
    "September",          // 8                   ^4
    "October",            // 9                   ^3
    "November",           // 10                  ^2
    "December"            // 11                  ^1
                          // 12 (nameOfMonths.Length)  ^0
};

• The 0 index means the first element of nameOfMonths[0].
• The ^0 index is the same as nameOfMonths[nameOfMonths.Length].
• The expression nameOfMonths[^0] does throw an exception, just as nameOfMonths[nameOfMonths.Length] does.
• For any number n, the index ^n is the same as nameOfMonths[nameOfMonths.Length-n].

You can retrieve the last month with the ^1 index.

Console.WriteLine("The last month is {0}", nameOfMonths[^1]);

Range

The System.Range struct represents a range that has start and end indexes.

• It represents a sub range of a sequence.
• The range operator .., which specifies the start and end of a range as its operands.
• The start of the range is inclusive, but the end of the range is exclusive, meaning the start is included in the range but the end isn't.
• The range [0..^0] represents the entire range, just as [0..nameOfMonths.Length] represents the entire range.

Let's consider the same array which contains all the names of months. The following code creates a subrange with the months "March", "April", and "May". It includes nameOfMonths[2] through nameOfMonths[5]. The element nameOfMonths[5] isn't in the range.

var names = nameOfMonths[2..5];

foreach (var name in names)
{
    Console.WriteLine(name);
}

You can also declare ranges as variables.

Range phrase = 1..3;

The following code creates a subrange with "November" and "December". It includes nameOfMonths[^2] and nameOfMonths[^1]. The end index nameOfMonths[^0] isn't included.

Range phrase = ^2..^0;
var names = nameOfMonths[phrase];

foreach (var name in names)
{
    Console.WriteLine(name);
}

The following code shows the ranges that are open ended for the start, end, or both.

var allMonths = nameOfMonths[..];       // contains all names.
var firstTwoMonths = nameOfMonths[..2]; // contains first two names i.e. January and February.
var lastThreeMonths = nameOfMonths[9..];     // contains the last three names.