Creating unique URLs in ASP.NET

Question

In my website, I need to create unique URLs that an admin user would use to send it to a group of users. The unique URL is created whenever an admin creates a new form. I understand I can use a guid to represent unique URLs, but I am looking for something shorter (hopefully around 4 characters, since it's easier to remember). How would I generate a unique URL in ASP.NET that would look like this:

http://mydomain.com/ABCD

I understand some of the URL shortener websites (like bit.ly) does something like this with a very short unique URL. Is there an algorithm I can use?

Is there any reason why you can't use an identity column in SQL, if you want them numbers to always be 4 characters long you can start the seed at "1000". There is no randomness to this, but it sounds like that's not an issue. — Zachary
– Zachary, Commented Feb 17, 2011 at 4:44

Bala R · Accepted Answer · 2011-02-17 04:33:03Z

1

How about something like

public static  string GetRandomString (int length)  
{  
 string charPool = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890";  
 StringBuilder sb = new StringBuilder();  
 Random rnd = new Random();

 while ((length--) > 0)  
 sb.Append(charPool[(int)(rnd.NextDouble() * charPool.Length)]);  

 return sb.ToString();  
}

and call

GetRandomString(4);

answered Feb 17, 2011 at 4:33

Bala R

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2 Comments

Matt Greer Over a year ago

rnd.Next(charPool.Length) would be better for the random call. But this approach has problems, in that it's possible it will generate the same random string more than once, leading to collisions.

CodesInChaos Over a year ago

Will produce collisions after around 46k outputs if length > 5, earlier for smaller lengths.

joshhendo · Accepted Answer · 2011-02-17 04:31:50Z

0

Just write an algorithm to select a certain number of characters from a GUID (e.g. the first 4 or 8 characters, every even character up to 4 or 8 characters.)

Be sure to check it against the database to make sure it isn't already in use, and if it is regenerate it. As a safeguard, maybe make a timeout (if it tries to generate 10 and they're all in use, give up,) but it's unlikely to use every possible combination.

answered Feb 17, 2011 at 4:31

joshhendo

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Comments

Jonathan Wood · Accepted Answer · 2011-02-17 04:42:43Z

0

I believe bit.ly performs a hash and then base64 encodes the result. You could do the same, although it'll be more than 4 characters. Be sure to add code that handles hashing collisions. You could append 1, 2, 3, etc. when the first hash is in use.

Another approach is to create a new table in a database. Every time you need a new URL, add a row to this table. You could use the PK as the URL value. This will give you up to 10,000 unique values using only four characters. Base64 encode for even more.

edited Feb 17, 2011 at 4:42

answered Feb 17, 2011 at 4:36

Jonathan Wood

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4 Comments

Neil N Over a year ago

I believe you mean Base 36, not 64 Bit

Jonathan Wood Over a year ago

@Neil Er... I guess I actually meant Base 64. Mistake fixed. Thanks.

Neil N Over a year ago

Base 64 wouldn't work either, as it is case sensitive, and URL's are not.

Jonathan Wood Over a year ago

@Neil Well, yeah, that could potentially break. But as an argument (which is what it would end up as if the URL was resolved with URL routing), it should work fairly reliably.

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