How Can I Use LINQ's Distinct() Method on Specific Object Properties?

Using LINQ's Distinct() for Unique Object Properties

LINQ's Distinct() method efficiently removes duplicate items from a sequence. However, you often need to identify uniqueness based on specific object properties, not the entire object.

Example: Unique Persons Based on ID

Let's say you have a Person class with an Id property. To get a list of unique persons based on their IDs, group the list by the Id and select the first person from each group:

<code class="language-csharp">List<Person> distinctPeople = allPeople
    .GroupBy(p => p.Id)
    .Select(g => g.First())
    .ToList();</code>

Handling Multiple Properties

To ensure uniqueness across multiple properties (e.g., Id and FavoriteColor), create an anonymous type as the grouping key:

<code class="language-csharp">List<Person> distinctPeople = allPeople
    .GroupBy(p => new { p.Id, p.FavoriteColor })
    .Select(g => g.First())
    .ToList();</code>

Important Note: Query Provider Compatibility

Some query providers (like older versions of Entity Framework Core) might throw exceptions if a group is empty. In such cases, replace First() with FirstOrDefault(). If FirstOrDefault() returns null, you'll need to handle that possibility in your code.

EF Core (Pre-Version 6) Solution

For EF Core versions before 6, a more robust approach is needed to handle potential empty groups. Refer to this Stack Overflow answer for a detailed solution: https://www.php.cn/link/7dd21654ce1c39ec7632d219e8e71f11 This provides a method that gracefully handles cases where a property combination doesn't have any matching objects.

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