How to generate a unique list of items from another list using LINQ in C#

Question

I have a list of items like so {One,Two,Three,One,Four,One,Five,Two,One} and I need a query that takes that list and generates a list based on only unique items, so the list returned would be {One,Two,Three,Four,Five}.

Answer 1

Use the Distinct operator:

var unique = list.Distinct();

Answer 2

The Distinct operator. There's an example at MSDN.

http://msdn.microsoft.com/en-us/library/bb348436.aspx

Answer 3

        var x = new string[] {"One", "Two", "Three", "One", "Four", "One", "Five", "Two", "One"}.ToList();
        var distinct = x.Distinct();

Answer 4

use distinct

    List<string> l = new List<string>
    {
        "One","Two","Three","One","Four","One","Five","Two","One"
    };

    var rootcategories2 = l.Distinct();

Answer 5

Aside from Distinct, as others have mentioned, you can also use a HashSet:

List<string> distinct = new HashSet<string>(list).ToList();

If you're using LINQ, though, go with Distinct.

Answer 6

It's worth noting Distinct() will use the default means of determining equality, which might not suit you if your list contains complex objects rather than primitives.

There is an overload that allows you to specify an IEqualityComparer for providing custom equality logic.

More details on how Distinct determines if two items are equal: http://msdn.microsoft.com/en-us/library/ms224763.aspx

Answer 7

Thanks for all your answers, I guess I mis-posed my question slightly. What I really have is a complex class, on which I want the comparison (for Distinct) based on a particular member of the class.

class ComplexClass
{
    public string Name{ get; set; }
    public string DisplayName{ get; set; }
    public int ComplexID{ get; set; }
}

List<ComplexClass> complexClassList = new List<ComplexClass>();

complexClassList.Add(new ComplexClass(){Name="1", DisplayName="One", ComplexID=1});
complexClassList.Add(new ComplexClass(){Name="2", DisplayName="Two", ComplexID=2});
complexClassList.Add(new ComplexClass(){Name="3", DisplayName="One", ComplexID=1});

// This doesn't produce a distinct list, since the comparison is Default
List<ComplexClass) uniqueList = complexClassList.Distinct();

class ComplexClassNameComparer : IEquatable<ComplexClass>
{
    public override bool Equals(ComplexClass x, ComplexClass y)
    {
        return (x.To.DisplayName == y.To.DisplayName);
    }

    public override int GetHashCode(ComplexClass obj)
    {
        return obj.DisplayName.GetHashCode();
    }
}

// This does produce a distinct list, since the comparison is specific
List<ComplexClass) uniqueList = Enumerable.Distinct(complexClassList , new ComplexClassNameComparer());