midLINQ
What is GroupJoin in LINQ?
Updated May 16, 2026
Short answer
GroupJoin creates hierarchical grouping between two sequences.
Deep explanation
It associates each element from outer sequence with matching inner elements.
Real-world example
User with list of their orders.
Common mistakes
- Confusing with Join which flattens results.
Follow-up questions
- Is GroupJoin one-to-many?
- What does it return?