My name
is
Jon Skeet

Get a list of duplicates from list based on properties comparison

I have a List<Demo>

public class Demo
{
    public Demo()
    {

    }

    public int Id { get; set; }
    public string Name { get; set; }
    public string Title { get; set; }
}

The Id property is unique for every record in the list.

How can I get a List<Demo> from the original List<Demo> with all the duplicates where the Name and Title are same.

What I did so far but what I get is one single record:

List<Demo> demo = new List<Demo>();

demo.Add(new Demo()
{
    Id = 1,
    Name = "Demo",
    Title = "A"
});

demo.Add(new Demo()
{
    Id = 2,
    Name = "Demo",
    Title = "A"
});

demo.Add(new Demo()
{
    Id = 3,
    Name = "Demo",
    Title = "A"
});

demo.Add(new Demo()
{
    Id = 4,
    Name = "Demo1",
    Title = "A"
});

demo.Add(new Demo()
{
    Id = 5,
    Name = "Demo2",
    Title = "A"
});

and then i am doing:

var duplicates = demo.GroupBy(t => new { t.Name, t.Title })
                     .Where(t => t.Count() > 1)
                     .Select(g => g.Key).ToList();

From the example above I should get a List<Demo> with the first 3 items where the ids are:1,2,3 because Name and Title are same.

It sounds like all you're missing is a SelectMany call. Currently you're creating all the appropriate groups and filtering down to groups with more than one entry - but if you want a single flat list, you need to flatten those groups back to their elements:

var duplicates = demo
    .GroupBy(t => new { t.Name, t.Title })
    .Where(t => t.Count() > 1)
    .SelectMany(x => x) // Flatten groups to a single sequence
    .ToList();

Note that this doesn't mean every entry in the resulting list will have the same name and title. It does mean that every entry will have a name/title combination in common with at least one other entry.

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