Distinct Linq filtering with additional criteria

Question

I have a list which contains duplicate item values (by ID), but with a different (or possibly equal) priority. Duplicate items with same or lower priority should be removed from the list.

For example:

var items = new {
   new { Id=2, Priority=3 },
   new { Id=4, Priority=4 },
   new { Id=1, Priority=4 },
   new { Id=2, Priority=5 },
   new { Id=4, Priority=4 }
};

RemoveDuplicates(items);

// items should now contain distinct values,
// with highest possible priority
var items = new {
   new { Id=1, Priority=4 }, // this one was unique
   new { Id=2, Priority=5 }, // this one was duplicate with higher priority
   new { Id=4, Priority=4 }, // this one was duplicate with same priority
};

Is it possible to do this using LINQ? I know I could sort the list by ID and then check adjacent items, but just wanted to check if this is possible.

(update: input values are not necessarily grouped by IDs)

Answer 1

        var items = new[] {
           new { Id=2, Priority=3 },
           new { Id=2, Priority=5 },
           new { Id=1, Priority=4 },
           new { Id=4, Priority=4 },
           new { Id=4, Priority=4 }
        };

        var deduped = items
            .GroupBy(item => item.Id)
            .Select(group => group.OrderByDescending(item => item.Priority).First())
            .OrderBy(item => item.Id);

Answer 2

The Distinct Extension Method returns the distinct elements from a sequence. You can provide an IEqualityComparer<TSource> to determine when two elements are equal. However, the method does not allow you to choose which of the two equal elements is used.

You can use the GroupBy Extension Method to group the list by ID and then select the element with the highest priority from each group:

var query = items.GroupBy(item => item.Id)
                 .Select(g => g.MaxBy(item => item.Priority))
                 .OrderBy(item => item.Id);

using MaxBy from MoreLINQ.