C# FirstOrDefault vs SingleOrDefault

Semantical Difference:

• FirstOrDefault returns a first item of potentially multiple (or default if none exists).
• SingleOrDefault assumes that there is a single item and returns it (or default if none exists). Multiple items are a violation of contract, an exception is thrown.

Performance Difference

• FirstOrDefault is usually faster, it iterates until it finds the element and only has to iterate the whole enumerable when it doesn't find it. In many cases, there is a high probability to find an item.
• SingleOrDefault needs to check if there is only one element and therefore always iterates the whole enumerable. To be precise, it iterates until it finds a second element and throws an exception. But in most cases, there is no second element.

Conclusion

• Use FirstOrDefault if you don't care how many items there are or when you can't afford checking uniqueness (e.g. in a very large collection). When you check uniqueness on adding the items to the collection, it might be too expensive to check it again when searching for those items.
• Use SingleOrDefault if you don't have to care about performance too much and want to make sure that the assumption of a single item is clear to the reader and checked at runtime.
• If you result set returns 1 record:
  • SingleOrDefault returns that record
  • FirstOrDefault returns that record
  If your result set returns many records:
  • SingleOrDefault throws an exception
  • FirstOrDefault returns the first record