Does slicing an array using Index/Range create a copy of the array

Question

With C# 8.0 seeing the introduction of the structs Index and Range we can now easily get a slice of an array doing something like

string[] baseArray = {"a","b", "c", "d", "e", "f"};

var arr = baseArray[1..^2];

Does slicing an array like this copy the array? Or does it do it without copying like ArraySegment<T>?

Your tests aren't really valid. All you're doing it creating the structures, not using them. For example, the Linq version just returns an IEnumerable but doesn't actually do anything. — DavidG
– DavidG, Commented May 24, 2019 at 9:15
@DavidG alright, thanks for pointing that out. Removed my test case. However the original question still stands wether or not it copies the array? — Remy
– Remy, Commented May 24, 2019 at 9:29

Cid · Accepted Answer · 2019-06-03 08:18:22Z

Try it yourself :

string[] baseArray = { "a", "b", "c", "d", "e", "f" };
var arr = baseArray[1..^2];
Debug.WriteLine(arr[0]);
Debug.WriteLine(baseArray[1]);
arr[0] = "hello";
Debug.WriteLine(arr[0]);
Debug.WriteLine(baseArray[1]);

Output

b
b
hello
b

We can conclude that the string array is copied.

However, if we are using an array of objects :

public class Foo
{
    public string Bar { get; set; }
}

Foo[] baseArray =
{
    new Foo { Bar = "a" },
    new Foo { Bar = "b" },
    new Foo { Bar = "c" },
    new Foo { Bar = "d" },
    new Foo { Bar = "e" },
    new Foo { Bar = "f" }
};

var arr = baseArray[1..^2];
Debug.WriteLine(arr[0].Bar);
Debug.WriteLine(baseArray[1].Bar);
arr[0].Bar = "hello";
Debug.WriteLine(arr[0].Bar);
Debug.WriteLine(baseArray[1].Bar);

arr[0] = new Foo { Bar = "World" };
Debug.WriteLine(arr[0].Bar);
Debug.WriteLine(baseArray[1].Bar);

This outputs

b
b
hello
hello
World
hello

Objects in the array aren't copied but referenced.

Setting another object in an array won't affect the other one.

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