FirstOrDefault vs. SingleOrDefault in LINQ: Key Differences and Use Cases

In this article, we explore the key differences between FirstOrDefault and SingleOrDefault in LINQ. You'll learn how each method behaves, the scenarios where they are best applied, and when to choose one over the other. By the end, you’ll gain clarity on how to use them effectively to write cleaner and more efficient LINQ queries in your C# applications.

Understanding the correct usage of these methods helps you avoid common runtime errors, improve application performance, and maintain predictable code behavior. Always perform null checks when working with reference types, and use SingleOrDefault() cautiously in performance-critical sections of your code.

Difference between FirstOrDefault() and SingleOrDefault()

If you’ve used C#, you’ve probably come across LINQ—it's a handy tool that makes working with data collections a lot easier. Two common LINQ methods you'll see are FirstOrDefault() and SingleOrDefault(). They might seem pretty similar at first, but choosing the wrong one can lead to weird bugs or slowdowns in your code.

In this short and simple guide, we’ll explain the key differences, show you when to use each one, and help you avoid common mistakes in real-life coding situations.

What is FirstOrDefault()?

FirstOrDefault() returns the first element of a sequence or the default value (e.g., null for reference types) if no element is found.

Use Cases

• When multiple results are expected, but you want just the first match.
• When the sequence can be empty.
• When performance is a concern and early exit is ideal.

Code Example

var firstProduct = products.FirstOrDefault(p => p.Price > 100);

What is SingleOrDefault()?

SingleOrDefault() returns exactly one element that matches the condition or the default value if no match is found. It throws an exception if more than one element is found.

var uniqueProduct = products.SingleOrDefault(p => p.Id == 101);

Use Cases

• When you expect only one matching result.
• Ideal for unique identifiers, like primary keys.
• Use it when data integrity is important and duplicates are unexpected.

⚖️ FirstOrDefault vs. SingleOrDefault

Feature	FirstOrDefault	SingleOrDefault
Returns	First match or default	Single match or default
Throws if multiple matches?	❌ No	✅ Yes
Use case	First matching item	Exactly one expected match
Performance	Stops at the first match	Scans all items to validate uniqueness

Performance Considerations

• FirstOrDefault() stops evaluating after the first match — faster on large datasets.
• SingleOrDefault() must scan the entire sequence to ensure there's only one match — slower if the collection is large.

Best Practices

• Use SingleOrDefault(): only when data constraints guarantee that at most one element can satisfy the condition.
• Prefer FirstOrDefault(): When multiple matches are possible and you only need the first occurrence, or can handle a missing result.
• Always perform a null check: after using either method when working with reference types to prevent NullReferenceException.
• Minimize the use of SingleOrDefault(): In performance-sensitive operations, it must evaluate the entire collection to ensure uniqueness.

Summary

In LINQ, both FirstOrDefault() and SingleOrDefault() are used to retrieve elements from a collection, but they serve different purposes. FirstOrDefault() returns the first matching element or a default value if none is found—ideal when multiple matches are possible. On the other hand, SingleOrDefault() ensures only one match exists and throws an exception if multiple results are found, making it suitable for unique identifiers or strict data constraints.