C# Linq：FirstOrDefault vs SingleOrDefault

In C#, both FirstOrDefault and SingleOrDefault are LINQ methods that operate on collections, but they serve different purposes:

FirstOrDefault

  • Purpose: Returns the first element in a collection that satisfies a specified condition, or the default value for the type if no such element is found.
  • Behavior:
    • If the collection contains multiple elements that satisfy the condition, it returns the first one.
    • If no elements satisfy the condition, it returns the default value (null for reference types, 0 for numeric types, etc.).
  • Use Case: When you're interested in getting the first match or a default value if none exist, and you don't care if there are multiple matches.

SingleOrDefault

  • Purpose: Returns the single element in a collection that satisfies a specified condition, or the default value for the type if no such element is found.
  • Behavior:
    • If the collection contains exactly one element that satisfies the condition, it returns that element.
    • If no elements satisfy the condition, it returns the default value.
    • If more than one element satisfies the condition, it throws an InvalidOperationException because the expectation is that there should be exactly one match.
  • Use Case: When you're expecting either one or zero matches, and multiple matches would indicate an error in your data or logic.

Summary

  • FirstOrDefault: Use when you want the first matching element or a default value, and multiple matches are acceptable.
  • SingleOrDefault: Use when you expect exactly one matching element or a default value, and multiple matches are an error.