Behaviour of negative array index in C#

Question

I'm newbie in C#. I know C and C++ language. Currently I have a C# related project. So, I just want to know basic concept about C#.

In C#, If I give negative array index, then What happens? Is it Undefined behaviour?

Like :

int [] arr = {1,2,3};
Console.WriteLine("{0}", arr[-1]);

Why not simply try it out? I think we can expect a 14k-user to at least know what kind of question you may ask on Stack and *how* to do so. — MakePeaceGreatAgain, Feb 03 '18 at 14:29

Jonathan Wood · Accepted Answer · 2018-02-03T14:07:59.927

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Your program will throw an IndexOutOfRangeException exception any time the index is out of the range of valid indexes for that array.

Had you taken a second to try it, you would've seen that for yourself.

edited Feb 03 '18 at 14:07

answered Feb 03 '18 at 14:00

Jonathan Wood

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score 1 · Answer 2 · edited Feb 03 '18 at 15:51

1

int [] arr = {1,2,3};

Compiler will transform the above syntactic sugar internally as

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

so arr length is calculated as 3 by the compiler automatically.

Compiler will not allow you define array of unknown size.

int[] arr=new int[];//compiler error,array creation must have size.

So below statement

Console.WriteLine(arr[-1]);

will throw an unhandled exception of type 'System.IndexOutOfRangeException'.

edited Feb 03 '18 at 15:51

Alex

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answered Feb 03 '18 at 14:08

Hameed Syed

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arr={1,2,3} internally will be transformed as int[].And also why -1 is not allowed because array index starts from 0 and thats because it uses pointers internally and below 0 is junk value and hence compiler warns from giving invalid indices.And also I am tying in mobile didnt see your answer before. – Hameed Syed Feb 03 '18 at 14:19

score 0 · Answer 3 · answered Feb 03 '18 at 14:05

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If you access an array out of its index range, you'll get an System.IndexOutOfRangeException. This exception you'll get for any negative index or any index larger or equal array.Length.

answered Feb 03 '18 at 14:05

milbrandt

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Behaviour of negative array index in C#

3 Answers3