When working with collections that are known to be non-empty, using First or Single is generally preferred over FirstOrDefault or SingleOrDefault.

Why is this an issue?

Using FirstOrDefault or SingleOrDefault on collections that are known to be non-empty is an issue due to:

• Code Clarity and intent: When you use FirstOrDefault or SingleOrDefault, it implies that the collection might be empty, which can be misleading if you know it is not. It can be confusing for other developers who read your code, making it harder for them to understand the actual constraints and behavior of the collection. This leads to confusion and harder-to-maintain code.
• Error handling: If the developer’s intend is for the collection not to be empty, using FirstOrDefault and SingleOrDefault can lead to subtle bugs. These methods return a default value (null for reference types and default for value types) when the collection is empty, potentially causing issues like NullReferenceException later in the code. In contrast, First or Single will throw an InvalidOperationException immediately if the collection is empty, making it easier to detect and address issues early in the development process.
• Code coverage: Potentially, having to check if the result is null, you introduces a condition that cannot be fully tested, impacting the code coverage.

Code examples

Noncompliant code example

var items = new List<int> { 1, 2, 3 };

int firstItem = items.FirstOrDefault(); // Noncompliant, this implies the collection might be empty, when we know it is not

Compliant solution

var items = new List<int> { 1, 2, 3 };

int firstItem = items.First(); // Compliant

Resources

Documentation

• Microsoft Learn - Single
• Microsoft Learn - First
• Microsoft Learn - SingleOrDefault
• Microsoft Learn - FirstOrDefault

Articles & blog posts

• First vs FirstOrDefault, Single vs SingleOrDefault: A High-level Look