How Can I Use LINQ to Get Distinct Objects Based on Specific Properties?

LINQ's Distinct() with Complex Object Properties

LINQ's Distinct() method provides an efficient way to remove duplicate elements from a collection. However, finding distinct elements based on one or more object properties requires a different approach.

Grouping Properties for Distinct Results

To obtain distinct results based on specific properties, use the GroupBy() and First() methods to create groups and select a representative from each group. For example:

List<Person> distinctPeople = allPeople
  .GroupBy(p => p.PersonId)
  .Select(g => g.First())
  .ToList();

In this scenario, the query groups all people by their PersonId property and selects the first person from each group. The result is a distinct list of people based on their PersonId.

Grouping Multiple Properties

If distinct results are needed based on multiple properties, modify the query as follows:

List<Person> distinctPeople = allPeople
  .GroupBy(p => new { p.PersonId, p.FavoriteColor })
  .Select(g => g.First())
  .ToList();

This query groups people by their PersonId and FavoriteColor properties, ensuring a distinct list based on both criteria.

Considerations

Some query providers may not guarantee that each group has at least one element. In such cases, consider using FirstOrDefault() instead of First(). Additionally, this technique may not be suitable for use with Entity Framework Core versions prior to 6. Consult alternative approaches at https://stackoverflow.com/a/66529949/8155 for compatibility in such environments.

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