C# Academy · Lesson

The Range Type and .. Operator

Take slices with range expressions.

Lesson 2 of 413 steps

The Range Type and .. Operator is a free C# Academy lesson on CoddyKit. This is lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, and your progress syncs across the web and the CoddyKit app. The C# Academy course includes 4 lessons in total.

Meet the Range Type

System.Range describes a slice of a sequence using a start and an end Index. It pairs perfectly with the range operator .. to express sub-sequences.

using System;

Range r = 1..3;
Console.WriteLine(r.Start + ".." + r.End); // 1..3

The .. (range) Operator

The range operator a..b builds a Range from index a (inclusive) to index b (exclusive). So 1..3 covers positions 1 and 2.

using System;

int[] data = { 10, 20, 30, 40, 50 };
int[] slice = data[1..3];
Console.WriteLine(string.Join(",", slice)); // 20,30

Start Inclusive, End Exclusive

The exclusive end mirrors how for loops work: the number of elements equals end - start. Range 2..5 yields three elements.

using System;

int[] data = { 0, 1, 2, 3, 4, 5, 6 };
int[] slice = data[2..5];
Console.WriteLine(slice.Length);          // 3
Console.WriteLine(string.Join(",", slice)); // 2,3,4

Open-Ended Ranges

You may omit either bound. ..3 means "from the start up to 3", and 2.. means "from 2 to the end". .. alone copies the whole sequence.

using System;

int[] d = { 10, 20, 30, 40, 50 };
Console.WriteLine(string.Join(",", d[..3])); // 10,20,30
Console.WriteLine(string.Join(",", d[2..])); // 30,40,50
Console.WriteLine(string.Join(",", d[..]));  // 10,20,30,40,50

Ranges with From-End Indices

Combine .. with ^ to slice relative to the end. ^2.. means "the last two elements".

using System;

int[] d = { 1, 2, 3, 4, 5 };
Console.WriteLine(string.Join(",", d[^2..]));  // 4,5
Console.WriteLine(string.Join(",", d[..^1]));  // 1,2,3,4 (drop last)

Range as a Stored Value

A Range is a value you can name and reuse across different collections of the same length convention.

using System;

Range middle = 1..^1; // drop first and last
int[] a = { 0, 1, 2, 3, 4 };
Console.WriteLine(string.Join(",", a[middle])); // 1,2,3

Range.GetOffsetAndLength

To resolve a range against a concrete length, call GetOffsetAndLength(length). It returns the start offset and the slice length, handling from-end indices for you.

using System;

Range r = ^3..^1;
var (offset, length) = r.GetOffsetAndLength(5);
Console.WriteLine(offset + " " + length); // 2 2

Range.All

The static Range.All is the same as .. and selects the entire sequence. It is handy when passing ranges as arguments.

using System;

Range whole = Range.All;
int[] d = { 9, 8, 7 };
Console.WriteLine(string.Join(",", d[whole])); // 9,8,7

Constructing Range Explicitly

Besides the operator, you can call the constructor or factory methods Range.StartAt and Range.EndAt for clarity.

using System;

Range a = new Range(1, 3);
Range b = Range.StartAt(2);
Range c = Range.EndAt(2);
Console.WriteLine(a + " | " + b + " | " + c);

Empty and Invalid Ranges

A range where start equals end produces an empty slice. A range whose start is past its end (after resolving) throws at slice time.

using System;

int[] d = { 1, 2, 3, 4 };
Console.WriteLine(d[2..2].Length); // 0 (empty)
try { var _ = d[3..1]; }
catch (ArgumentOutOfRangeException) { Console.WriteLine("invalid range"); }

Putting It Together

Ranges make text and array processing concise. Here we grab a window in the middle of an array without any index arithmetic.

using System;

int[] readings = { 5, 7, 9, 11, 13, 15 };
int[] window = readings[1..^1];
Console.WriteLine(string.Join(",", window)); // 7,9,11,13

Quick Check

Make sure you understand the range operator semantics.

Answer it in the interactive lesson

Recap

You learned that System.Range describes a slice with the .. operator.

Start is inclusive, end is exclusive: count = end - start.
Bounds are optional: ..3, 2.., ...
.. combines with ^ for from-end slicing.
GetOffsetAndLength(length) resolves a range to concrete numbers.

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What will I learn in “The Range Type and .. Operator”?

Take slices with range expressions. You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

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