Substring index and length must refer to a location within the string

Question

I have a string that looks like

string url = "www.example.com/aaa/bbb.jpg";

"www.example.com/" is 18 fixed in length. I want to get the "aaa/bbb" part from this string (The actual url is not example nor aaa/bbb though, the length may vary)

so here's what I did:

string newString = url.Substring(18, url.Length - 4);

Then I got the exception: index and length must refer to a location within the string. What's wrong with my code and how to fix it?

The arguments to substring are (startIndex, length). I'm not sure where you're getting 18 and Length-4 from for the substring you want. — Joe Castro, Jun 21 '12 at 23:27
Need to also subtract the 18 to account for the initial "www.xxx.com/" EDIT: I mean, 12. Your 18 should be 12 :) — Chris Sinclair, Jun 21 '12 at 23:27
Can you use the System.Uri (http://msdn.microsoft.com/en-us/library/system.uri.aspx) class for this instead? — Andrew Kennan, Jun 21 '12 at 23:38

score 70 · Accepted Answer · edited Jun 09 '22 at 19:24

The second parameter in Substring is the length of the substring, not the end index (in other words, it's not the length of the full string).

You should probably include handling to check that it does indeed start with what you expect, end with what you expect, and is at least as long as you expect. And then if it doesn't match, you can either do something else or throw a meaningful error.

Here's some example code that validates that url contains your strings, that also is refactored a bit to make it easier to change the prefix/suffix to strip:

var prefix = "www.example.com/";
var suffix = ".jpg";
string url = "www.example.com/aaa/bbb.jpg";

if (url.StartsWith(prefix) && url.EndsWith(suffix) && url.Length >= (prefix.Length + suffix.Length))
{
    string newString = url.Substring(prefix.Length, url.Length - prefix.Length - suffix.Length);
    Console.WriteLine(newString);
}
else
    //handle invalid state

string newString = url.Substring(18, url.Length - 18); – Nirbhay Kumar Mar 09 '21 at 06:25 — Nirbhay Kumar, Mar 09 '21 at 06:25

MAV · Answer 2 · 2012-06-21T23:51:42.227

8

Your mistake is the parameters to Substring. The first parameter should be the start index and the second should be the length or offset from the startindex.

string newString = url.Substring(18, 7);

If the length of the substring can vary you need to calculate the length.

Something in the direction of (url.Length - 18) - 4 (or url.Length - 22)

In the end it will look something like this

string newString = url.Substring(18, url.Length - 22);

edited Jun 21 '12 at 23:51

answered Jun 21 '12 at 23:28

MAV

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The length of "aaa/bbb" part varies from time to time so it's not exactly 7. But thanks for answering my question. upvoted your answer :) – Manto Jun 21 '12 at 23:37
22 is correct, because you want to know the length, not the index – walther Jun 21 '12 at 23:45

score 7 · Answer 3 · edited Sep 04 '16 at 12:32

7

How about something like this :

string url = "http://www.example.com/aaa/bbb.jpg";
Uri uri = new Uri(url);
string path_Query = uri.PathAndQuery;
string extension =  Path.GetExtension(path_Query);

path_Query = path_Query.Replace(extension, string.Empty);// This will remove extension

edited Sep 04 '16 at 12:32

Kenster

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answered Jun 21 '12 at 23:51

HatSoft

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1

Good solution relying solely on library functions instead of hacky string indexing which makes it more readable and more robust. – Good Night Nerd Pride Sep 04 '16 at 19:47

score 4 · Answer 4 · edited Sep 04 '16 at 12:31

4

You need to find the position of the first /, and then calculate the portion you want:

string url = "www.example.com/aaa/bbb.jpg";
int Idx = url.IndexOf("/");
string yourValue = url.Substring(Idx + 1, url.Length - Idx - 4);

edited Sep 04 '16 at 12:31

Kenster

23,465
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answered Jun 21 '12 at 23:39

Ken White

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score 2 · Answer 5 · answered Jun 21 '12 at 23:36

2

Try This:

 int positionOfJPG=url.IndexOf(".jpg");
 string newString = url.Substring(18, url.Length - positionOfJPG);

answered Jun 21 '12 at 23:36

Bishnu Paudel

2,083
1
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score 2 · Answer 6 · answered Jun 21 '12 at 23:44

2

string newString = url.Substring(18, (url.LastIndexOf(".") - 18))

answered Jun 21 '12 at 23:44

Bastardo

4,144
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score 0 · Answer 7 · edited Sep 04 '16 at 12:32

0

Here is another suggestion. If you can prepend http:// to your url string you can do this

  string path = "http://www.example.com/aaa/bbb.jpg";
  Uri uri = new Uri(path);            
  string expectedString = 
      uri.PathAndQuery.Remove(uri.PathAndQuery.LastIndexOf("."));

edited Sep 04 '16 at 12:32

Kenster

23,465
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answered Jun 21 '12 at 23:47

Prashanth Thurairatnam

4,353
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score 0 · Answer 8 · answered Nov 11 '20 at 13:45

0

You need to check your statement like this :

string url = "www.example.com/aaa/bbb.jpg";
string lenght = url.Lenght-4;
if(url.Lenght > 15)//eg 15
{
 string newString = url.Substring(18, lenght);
}

answered Nov 11 '20 at 13:45

Engin Karataş

45
4

Muharrem Ekimci · Answer 9 · 2022-09-14T15:32:22.387

0

Can you try this ?

string example = url.Substring(0,(url.Length > 18 ? url.Length - 4 : url.Length))

edited Sep 14 '22 at 15:32

answered Sep 07 '22 at 07:37

Muharrem Ekimci

1
1

As it’s currently written, your answer is unclear. Please [edit] to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers [in the help center](/help/how-to-answer). – Community Sep 12 '22 at 06:01

Substring index and length must refer to a location within the string

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