Checking for duplicates in a List of Objects C#

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