How to Use LINQ's Distinct() Method on Specific Properties of Complex Objects?

Extracting Unique Records Based on Specific Properties Using LINQ

LINQ's Distinct() method efficiently removes duplicate elements from a collection. However, when dealing with complex objects (e.g., a List<T>), directly using Distinct() on the entire object might not yield the desired result if you only need uniqueness based on certain properties. This article demonstrates how to achieve this using GroupBy and Select.

Leveraging GroupBy for Property-Based Uniqueness

The key to obtaining distinct records based on specific properties is to utilize LINQ's GroupBy method. This groups elements sharing the same property (or combination of properties) and then allows you to select a representative from each group.

Let's illustrate with a List<Person>, where each Person object has properties like Id and Name:

<code>Person1: Id=1, Name="Test1"
Person2: Id=1, Name="Test1"
Person3: Id=2, Name="Test2"</code>

To retrieve distinct Person objects based solely on the Id property, use the following LINQ query:

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

Here, GroupBy(p => p.Id) groups the Person objects by their Id values. Select(g => g.First()) then selects the first element from each group, effectively giving you one Person object per unique Id.

Extending to Multiple Properties

This technique easily extends to multiple properties. To group by both Id and FavoriteColor:

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

This groups Person objects based on the combination of Id and FavoriteColor, ensuring uniqueness across both properties. Again, Select(g => g.First()) picks a single representative from each group.

Important Considerations

This approach assumes your query provider can handle potentially empty groups. If this isn't guaranteed, consider using FirstOrDefault() instead of First() to gracefully handle such cases.

For older versions of Entity Framework Core (prior to version 6), alternative methods might be necessary. Refer to this Stack Overflow answer for guidance: https://www.php.cn/link/7dd21654ce1c39ec7632d219e8e71f11

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