C# Substring Extensions

Most of the time when I'm using Substring for something, what I'm actually trying to achieve is getting the contents of the string between two other well-known parts. For example, in a query string I might have something like this: http://mysite.com/page?user=mickey&location=Disney&bestie=Donald. If I want to get the location from that string (and yes, I know there are utilities specifically designed to get values out of query strings, but this is the best example I could come up with on short notice so get over it) I'd have to find "location" in the string, then find the next & and then get the contents between them.

Unfortunately, the out-of-the-box versions of Substring only allow us to specify the beginning index or the beginning index and a length. If I don't know the length then I have to do some mucking about with the contents and... you know what, you've probably dealt with this before. I finally decided to write some extension methods for Substring (well, for strings) that take the string I'm searching for and give me back what I want.

   1:  public static class StringExtensions

   2:  {

   3:      public static string Substring(this string input, string searchText, StringComparison comparisonType = StringComparison.InvariantCultureIgnoreCase)

   4:      {

   5:          // find the first occurrence of the search text

   6:          var index = input.IndexOf(searchText, comparisonType);

   7:  

   8:          // either return null or the entire string that comes after the search text

   9:          // append the length of the search string to the index so the search string isn't included in the result

  10:          return index == -1 ? null : input.Substring(index + searchText.Length);

  11:      }

  12:  

  13:      public static string Substring(this string input, string primarySearchText, string secondarySearchText,

  14:          StringComparison comparisonType = StringComparison.InvariantCultureIgnoreCase)

  15:      {

  16:          // find the first occurrence of the primary search text

  17:          var index = input.IndexOf(primarySearchText, comparisonType);

  18:  

  19:          if (index == -1)

  20:          {

  21:              // if the primary search text doesn't exist in the string, just return null

  22:          }

  23:  

  24:          // append the length of the primary search string to the index so the search string isn't included in the result

  25:          index += primarySearchText.Length;

  26:  

  27:          // find the first occurrence of the secondary search text

  28:          var searchUntilIndex = input.IndexOf(secondarySearchText, index, comparisonType);

  29:          // if the secondary search text doesn't exist in the string (after the primary search text occurs),

  30:          // return the entire string that comes after the primary search text

  31:          // if the secondary search text does exist in the string after the primary search text occurs,

  32:          // return whatever is between the primary search text and the secondary search text

  33:          var length = searchUntilIndex == -1 ? input.Length - index : searchUntilIndex - index;

  34:  

  35:          return input.Substring(index, length);

  36:      }

  37:  }

These new methods let me do this:

var location = input.Substring("location=", "&");

and what I'll get back from that call is "Disney" (without the quotes, of course).

I've probably written some form of this over a dozen times. At least now I've saved it for future reference so I can copy/paste it next time I need it.