Question

I have a List<> of custom objects. This custom type has a property called Name which should be unique among the list. In other words, no 2 items items in the list should have the same value for their Name property.

When I validate this list, I want to retrieve the offending items. Is there a Linq operation which will allow me to do this?

I want to have something like

listOfItems.Where(x => x.Name.Equals(/*anything else in this list with the same value for name */)

Basically, I am trying to avoid checking the entire list against every item in the list (in a nested foreach):

private IList<ICustomObject> GetDuplicatedTypeNames(IList<ICustomObjects> customObjectsToFindDuplicatesIn)
    {
        var duplicatedList = new List<ICustomObject>();

        foreach(var customObject in customObjectsToFindDuplicatesIn)
            foreach(var innerCustomObject in customObjectsToFindDuplicatesIn)
                if (customObject == innerCustomObject && customObject .Name.Equals(innerCustomObject.Name))
                    duplicatedList.Add(customObject);

        return duplicatedList;
    }

(EDIT) NOTE: I am constrained to using a List<> by domain rules and using a Dictionary<> is not an option.

Answer 1

Get names of duplicates:

 var duplicates = listOfItems
        .GroupBy(i => i.Name)
        .Where(g => g.Count() > 1)
        .Select(g => g.Key);

Edit: Get duplicate items:

var duplicates = listOfItems
    .GroupBy(i => i.Name)
    .Where(g => g.Count() > 1)
    .SelectMany(g => g);

Answer 2

Why not use a Dictionary instead of a List then, with the Name property as the Key? That way, you can t add duplicate names to the collection at all, as an exception would be thrown.

Furthermore, you can use the ContainsKey method to test if a name is already in the Dictionary before adding it.

The advantage of this approach is that it s much faster than scanning the List for duplicates.

Answer 3

This will return a list of objects

class foo
{
    public string Name { get; set; }

    public override string ToString()
    {
        return Name;
    }
}

class fooEqualityComparer : IEqualityComparer<foo>
{
    public bool Equals(foo x, foo y)
    {
        if (x == null || y == null)
            return false;
        return x.Name == y.Name;
    }

    public int GetHashCode(foo obj)
    {
        return obj.Name.GetHashCode();
    }
}


var duplicates = listOfItems
.GroupBy(x => x, new fooEqualityComparer())
.Where(g => g.Count() > 1)
.SelectMany(g => g);

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