.NET collection that throws an exception when a duplicate is added

Question

Is there a collection (apart from Dictionary) in the .NET framework (3.5) that throws an exception when a duplicate is added?

HashSet does not throw an exception here:

HashSet<string> strings = new HashSet<string>();
strings.Add("apple");
strings.Add("apple");

Whereas the Dictionary does:

Dictionary<string, string> dict = new Dictionary<string, string>();
dict.Add("dude", "dude");
dict.Add("dude", "dude"); //throws exception

EDIT: Is there a collection without (Key, Value) that does this? I also want AddRange if possible...

I rolled my own:

public class Uniques<T> : HashSet<T>
{

    public Uniques()
    { }

    public Uniques(IEnumerable<T> collection)
    {
        AddRange(collection);
    }

    public void Add(T item)
    {
        if (!base.Add(item))
        {
            throw new ArgumentException("Item already exists");
        }
    }


    public void AddRange(IEnumerable<T> collection)
    {
        foreach (T item in collection)
        {
            Add(item);
        }
    }
}

Okay I see your edit, I'm deleting my original answer. To answer your edit, no. — ParmesanCodice, Nov 05 '09 at 10:28
Shouldn't you add the 'new' keyword to the 'Add' method signature, for it hides the inherited member HashSet.Add(T). — Gerard, Oct 01 '10 at 09:01
why dont you add extension methods to HashSet ? like AddRange/RemoveMany will be like Linq (like saying 'Linq') — Tomer W, Sep 06 '12 at 07:04

score 19 · Accepted Answer · answered Nov 05 '09 at 09:58

19

But the HashSet.Add method returns false if the value already is present - isn't that enough?

HashSet<string> set = new HashSet<string>();
...
if (!set.Add("Key"))
    /* Not added */

answered Nov 05 '09 at 09:58

Björn

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2

It allows you to add without the cost of checking Contains before, two operations in one call. There is no TryAdd on the Dictionary<,>. – Guillaume Nov 05 '09 at 10:21
I prefer it but it's strange as ICollection.Add returns void and others collection that disallow duplicates throw... – Guillaume Nov 05 '09 at 10:52
Ok, I sort of wanted AddRange functionality. Thanks – geejay Nov 05 '09 at 10:52
Oops... it does indeed. – alexkovelsky Dec 12 '19 at 22:59

score 0 · Answer 2 · answered Nov 05 '09 at 17:28

If you're looking for AddRange style functionality, look at C5. The collections in the C5 family have a lot more functionality exposed in their interfaces, including a function AddAll that takes an enumerable, adding all the items in the enumerable to the collection in turn.

EDIT: Also note that the C5 collections implement the System.Collections.Generic ICollection and IList interfaces where appropriate, so can be used as the implementation even in systems that expect those interfaces.

score 0 · Answer 3 · edited May 23 '17 at 12:01

To add to Bjorn's answer, if you also want a function like IList.AddRange type of function with HashSet<T>, you can use HashSet<T>.UnionWith (from MSDN):

HashSet(T).UnionWith Method

Modifies the current HashSet object to contain all elements that are present in itself, the specified collection, or both.

public void UnionWith(
    IEnumerable<T> other
)

Only problem might be: I'm pretty sure this requires the .NET Framework 3.5 SP1 and up.

.NET collection that throws an exception when a duplicate is added

3 Answers3

HashSet(T).UnionWith Method