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Case-Insensitive List Search

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 => x.IndexOf(keyword, 
                       StringComparison.OrdinalIgnoreCase) >= 0).Count > 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


p
pbz

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.


Definitely the best answer out here. :)
Enumerable.Contains (what you're referencing) hasn't been around since .NET 2.0. There is no List.Contains that has the overload you're using.
@AdamSills right. There is no such contains method in List. And if it's a lazy collection, than it can iterate it a couple of times as other Enumerable methods do. Imho, this method shouldn't be used for such cases, as it's not so logical for that case.
I was not seeing this overload at first either, but you need to add using System.Linq then it appears.
The StringComparer class has been around since 2.0, but that overload of Contains was introduced in 3.5. msdn.microsoft.com/en-us/library/bb339118(v=vs.110).aspx
A
Adam Sills

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 => 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 => s.Equals(keyword, StringComparison.OrdinalIgnoreCase) ) )
    Console.WriteLine("found in list");

Just to add, in a few quick tests, it seems that the first method is around 50% faster. Maybe someone else can confirm/deny that.
As of .NET 2.0, this is now easily done - look at shaxby's answer below.
The Contains method shaxby's referencing (that has an overload that takes an IEqualityComparer) is part of LINQ, so it certainly hasn't been available since .NET 2.0. Just the StringComparer class has been around for a while. List doesn't have that method, nor does ArrayList or StringCollection (things that he could have easily been referencing as his 'list').
Well, since I actually needed the index, this was definitely the best answer for me.
The first solution should use List<>.Exists(Predicate<>) instance method. Also note that if the list contains null entries, this can blow up. In that case it is more safe to say keyword.Equals(x, StringComparison.OrdinalIgnoreCase) than x.Equals(keyword, StringComparison.OrdinalIgnoreCase) (if you can guarantee that the keyword is never null).
L
Lance Larsen - Microsoft MVP

Based on Adam Sills answer above - here's a nice clean extensions method for Contains... :)

///----------------------------------------------------------------------
/// <summary>
/// Determines whether the specified list contains the matching string value
/// </summary>
/// <param name="list">The list.</param>
/// <param name="value">The value to match.</param>
/// <param name="ignoreCase">if set to <c>true</c> the case is ignored.</param>
/// <returns>
///   <c>true</c> if the specified list contais the matching string; otherwise, <c>false</c>.
/// </returns>
///----------------------------------------------------------------------
public static bool Contains(this List<string> list, string value, bool ignoreCase = false)
{
    return ignoreCase ?
        list.Any(s => s.Equals(value, StringComparison.OrdinalIgnoreCase)) :
        list.Contains(value);
}

P
Paul-Sebastian

You can use StringComparer static variants with the Contains overload from LINQ for example, like this:

using System.Linq;

var list = new List<string>();
list.Add("cat");
list.Add("dog");
list.Add("moth");

if (list.Contains("MOTH", StringComparer.OrdinalIgnoreCase))
{
    Console.WriteLine("found");
}

As long as you add "using System.Linq", otherwise you won't see that overload for .Contains.
d
dontbyteme

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<string> source, string toCheck, StringComparison comp)
{
    return
       source != null &&
       !string.IsNullOrEmpty(toCheck) &&
       source.Any(x => string.Compare(x, toCheck, comp) == 0);
}

I
Ilya Kogan

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 => x.IndexOf(keyword, 
                   StringComparison.OrdinalIgnoreCase) >= 0).Count > 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.


H
Himanshu Chopra

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<Book> listToSearch = new List<Book>()
   {
        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<string>()
            {
                "test", "b"
            }; 

foreach (var itemtoremove in blackList)
    {
        listToSearch.RemoveAll(p => p.Source.ToLower().Contains(itemtoremove.ToLower()) || p.Source.ToLower().Contains(itemtoremove.ToLower()));
    }


return listToSearch.ToList();

M
Monkey in Pajamas

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<string> ItemsList, string searchItem)
{
    List<string> lowercaselist = new List<string>();

    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!


why produce a second list? That's not very efficient. for(var i = 0; i < itemsList.Count; i++) { if (item.ToLower() == searchItem.ToLower()) { return i } }
I guess we will never know.