Case-Insensitive List Search

Question

I have a list testList that contains a bunch of strings  I would like to add a new string into the testList only if it doesn t already exist in the list  Therefore  I need to do a case-insensitive search of the list and make it efficient  I can t use Contains because that doesn t take into account the casing  I also don t want to use ToUpper ToLower for performance reasons  I came across this method  which works       if testList FindAll x   gt  x IndexOf keyword                          StringComparison OrdinalIgnoreCase   gt   0  Count  gt  0         Console WriteLine  Found in list      This works  but it also matches partial words  If the list contains  goat   I can t add  oat  because it claims that  oat  is already in the list  Is there a way to efficiently search lists in a case insensitive manner  where words have to match exactly  thanks

User · Accepted Answer

Instead of String IndexOf  use String Equals to ensure you don t have partial matches  Also don t use FindAll as that goes through every element  use FindIndex  it stops on the first one it hits    if testList FindIndex x   gt  x Equals keyword        StringComparison OrdinalIgnoreCase       -1       Console WriteLine  Found in list       Alternately use some LINQ methods  which also stops on the first one it hits   if  testList Any  s   gt  s Equals keyword  StringComparison OrdinalIgnoreCase          Console WriteLine  found in list

User · Answer

You re checking if the result of IndexOf is larger or equal 0  meaning whether the match starts anywhere in the string  Try checking if it s equal to 0   if  testList FindAll x   gt  x IndexOf keyword                      StringComparison OrdinalIgnoreCase   gt   0  Count  gt  0     Console WriteLine  Found in list      Now  goat  and  oat  won t match  but  goat  and  goa  will  To avoid this  you can compare the lenghts of the two strings   To avoid all this complication  you can use a dictionary instead of a list  They key would be the lowercase string  and the value would be the real string  This way  performance isn t hurt because you don t have to use ToLower for each comparison  but you can still use Contains

User · Answer

You can use StringComparer       var list   new List lt string gt         list Add  cat        list Add  dog        list Add  moth         if  list Contains  MOTH   StringComparer OrdinalIgnoreCase                 Console WriteLine  found

User · Answer

Based on Adam Sills answer above - here s a nice clean extensions method for Contains           ----------------------------------------------------------------------      lt summary gt      Determines whether the specified list contains the matching string value      lt  summary gt       lt param name  list  gt The list  lt  param gt       lt param name  value  gt The value to match  lt  param gt       lt param name  ignoreCase  gt if set to  lt c gt true lt  c gt  the case is ignored  lt  param gt       lt returns gt         lt c gt true lt  c gt  if the specified list contais the matching string  otherwise   lt c gt false lt  c gt        lt  returns gt     ---------------------------------------------------------------------- public static bool Contains this List lt string gt  list  string value  bool ignoreCase   false        return ignoreCase           list Any s   gt  s Equals value  StringComparison OrdinalIgnoreCase             list Contains value

User · Answer

Below is the example of searching for a keyword in the whole list and remove that item   public class Book     public int BookId   get  set      public DateTime CreatedDate   get  set      public string Text   get  set      public string Autor   get  set      public string Source   get  set        If you want to remove a book that contains some keyword in the Text property  you can create a list of keywords and remove it from list of books   List lt Book gt  listToSearch   new List lt Book gt                 new Book                BookId   1              CreatedDate   new DateTime 2014  5  27               Text     test voprivreda                  Autor    abc               Source    SSSS                      new Book                BookId   2              CreatedDate   new DateTime 2014  5  27               Text    here you go                  Autor    bcd               Source    SSSS                      var blackList   new List lt string gt                                   test    b                   foreach  var itemtoremove in blackList                listToSearch RemoveAll p   gt  p Source ToLower   Contains itemtoremove ToLower       p Source ToLower   Contains itemtoremove ToLower              return listToSearch ToList

User · Answer

I realise this is an old post  but just in case anyone else is looking  you can use Contains by providing the case insensitive string equality comparer like so   using System Linq           if  testList Contains keyword  StringComparer OrdinalIgnoreCase         Console WriteLine  Keyword Exists        This has been available since  net 2 0 according to msdn

User · Answer

Based on Lance Larsen answer - here s an extension method with the recommended string Compare instead of string Equals     It is highly recommended that you use an overload of String Compare that takes a StringComparison parameter    Not only do these overloads allow you to define the exact comparison behavior you intended  using them will also make your code more readable for other developers   Josh Free   BCL Team Blog    public static bool Contains this List lt string gt  source  string toCheck  StringComparison comp        return        source    null  amp  amp          string IsNullOrEmpty toCheck   amp  amp         source Any x   gt  string Compare x  toCheck  comp     0

User · Answer

I had a similar problem  I needed the index of the item but it had to be case insensitive  i looked around the web for a few minutes and found nothing  so I just wrote a small method to get it done  here is what I did   private static int getCaseInvariantIndex List lt string gt  ItemsList  string searchItem        List lt string gt  lowercaselist   new List lt string gt          foreach  string item in ItemsList                lowercaselist Add item ToLower                return lowercaselist IndexOf searchItem ToLower         Add this code to the same file  and call it like this   int index   getCaseInvariantIndexFromList ListOfItems  itemToFind     Hope this helps  good luck

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