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What is the difference between First, FirstOrDefault, Single and SingleOrDefault?

C# LINQ Basic Fresher1–2 Yrs Conceptual 1 min read

Quick Answer

First — first match, throws if none. FirstOrDefault — first match or default(T) if none. Single — exactly one match, throws if zero or more than one. SingleOrDefault — zero or one, throws if more than one.

First gives you the first matching element and throws if there are none. FirstOrDefault returns null or the default instead of throwing. Single insists there's exactly one — it throws both if there are zero and if there are two or more. SingleOrDefault allows zero but still throws on more than one. I use Single for a lookup by primary key because it also asserts my uniqueness assumption, and FirstOrDefault with an OrderBy when several matches are fine and I just want the top one.

  • Use Single/SingleOrDefault when the data model guarantees uniqueness (a lookup by primary key) — it also validates that assumption and fails loudly if it's violated.
  • Use First/FirstOrDefault when multiple matches are legitimate and you just want one (usually with an OrderBy).
  • FirstOrDefault on a reference type returns null; on a value type it returns 0/false — check accordingly.
  • In EF, Single can pull an extra row to check the “more than one” condition; First emits TOP 1.
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