Can a List of String be told to check for duplicates?

Question

Does a List have a property or mechanism by which I can prevent duplicate entries from being added to it, or do I have to search the list for it each time?

IOW, it would be much nicer just to go:

List<String> AAppsToDisplay = new List<String>();
AAppsToDisplay.DuplicatesAllowed = false;
...
while (odr.Read()) 
{
  AAppsToDisplay.Add(odr.GetString(0)); 
}

than to have to do this:

List<String> AAppsToDisplay = new List<String>();
. . .
String s = String.Empty;
while (odr.Read()) 
{
  s = odr.GetString(0);
  if (! AAppsToDisplay.Contains(s))
  {
    AAppsToDisplay.Add(s);  
  }
}

Do you require both *ordering* and being *free of duplicates*? — Eric Lippert, Mar 29 '12 at 23:15
Here you go Clay: http://stackoverflow.com/questions/918742/efficient-list-of-unique-strings-c-sharp none the less I'd still be interested in Eric's advice — Jeremy Thompson, Mar 29 '12 at 23:17
Just clarifying Eric's comment: ordering refers to items being stored in the order they were added, rather than a (basically) arbitrary order. If you do not need the list to be ordered, use a `HashSet`. — Kendall Frey, Mar 29 '12 at 23:18

Anthony Pegram · Accepted Answer · 2012-03-29T23:25:49.150

Use a more appropriate tool for the job instead, HashSet<string>.

var hashset = new HashSet<string>();

// in loop
hashset.Add(value);

If you were to use Contains on the HashSet, it would be a more optimal solution than using it on List<string>, particularly if the set grew particularly large. However, if it is permissible within your business requirement to silently discard duplicates, then you do not need to. The Add method returns a boolean indicating if the value was added. Duplicates never make it into the set.

It is important to note that HashSet<T> is not documented to preserve insertion order. You may observe results in the same order of insertion, and that may be impacted by your particular usage, but you should not rely upon it.

score 1 · Answer 2 · answered Mar 29 '12 at 23:16

1

You could use an extension method to the List class:

public static class StringListExtensions {
    public static bool AddIfNotPresent(this List<string> list, string newString) {
        if (!list.Contains(newString)) {
            this.Add(newString);
            return true;
        }
        return false;
    }
}

answered Mar 29 '12 at 23:16

Paul Grimshaw

19,894
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Why don't you like the HashSet? – Adrian Iftode Mar 29 '12 at 23:17
I know the HashSet is better, however the extension could suit if you have used this class in many places, or it's in a third party dll etc. – Paul Grimshaw Mar 29 '12 at 23:20
1

It's amazing how many different ways there are to skin a cat, but now I'm hungry for corned beef hash. – B. Clay Shannon-B. Crow Raven Mar 29 '12 at 23:42

score 1 · Answer 3 · answered Mar 29 '12 at 23:17

1

You could use System.Linq library and go

List<String> AAppsToDisplay = new List<String>();
...
while (odr.Read()) 
{
  AAppsToDisplay.Add(odr.GetString(0)); 
}

//new code
AAppsToDisplay = AAppsToDisplay.Distinct().ToList();

answered Mar 29 '12 at 23:17

m.t.bennett

1,290
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I have never heard of it, just copied and pasted from above sorry - have removed it – m.t.bennett Mar 29 '12 at 23:25

Can a List of String be told to check for duplicates?

3 Answers3