Checking for duplicates in a List of Objects C#

Question

I am looking for a really fast way to check for duplicates in a list of objects. I was thinking of simply looping through the list and doing a manual comparison that way, but I thought that linq might provide a more elegant solution... Suppose I have an object... ``` public class dupeCheckee { public string checkThis { get; set; } public string checkThat { get; set; } dupeCheckee(string val, string val2) { checkThis = val; checkThat = val2; } } ``` And I have a list of those objects ``` List dupList = new List(); dupList.Add(new dupeCheckee("test1", "value1")); dupList.Add(new dupeCheckee("test2", "value1")); dupList.Add(new dupeCheckee("test3", "value1")); dupList.Add(new dupeCheckee("test1", "value1"));//dupe dupList.Add(new dupeCheckee("test2", "value1"));//dupe... dupList.Add(new dupeCheckee("test4", "value1")); dupList.Add(new dupeCheckee("test5", "value1")); dupList.Add(new dupeCheckee("test1", "value2"));//not dupe ``` I need to find the dupes in that…

Accepted Answer

You need to reference System.Linq (e.g. `using System.Linq`)

then you can do

```
var dupes = dupList.GroupBy(x => new {x.checkThis, x.checkThat})
                   .Where(x => x.Skip(1).Any());
```

This will give you groups with all the duplicates

The test for duplicates would then be

```
var hasDupes = dupList.GroupBy(x => new {x.checkThis, x.checkThat})
                   .Where(x => x.Skip(1).Any()).Any();
```

or even call `ToList()` or `ToArray()` to force the calculation of the result and then you can both check for dupes and examine them.

eg..

```
var dupes = dupList.GroupBy(x => new {x.checkThis, x.checkThat})
                   .Where(x => x.Skip(1).Any()).ToArray();
if (dupes.Any()) {
  foreach (var dupeList in dupes) {
    Console.WriteLine(string.Format("checkThis={0},checkThat={1} has {2} duplicates",
                      dupList.Key.checkThis, 
                      dupList.Key.checkThat,
                      dupList.Count() - 1));
  }

}
```

Alternatively

```
var dupes = dupList.Select((x, i) => new { index = i, value = x})
                   .GroupBy(x => new {x.value.checkThis, x.value.checkThat})
                   .Where(x => x.Skip(1).Any());
```

Which give you the groups which each item per group stores the original index in a property `index` and the item in the property `value`

Checking for duplicates in a List of Objects C#

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