BulkMerge (Upsert) in EF Core: How to Insert-or-Update Without the Headache

This article addresses the common challenge of performing upsert operations (insert-or-update) in Entity Framework Core, particularly for high-volume data imports. It explains why EF Core lacks a built-in upsert method and why naive approaches using loops and FirstOrDefault cause severe performance degradation. The post explores practical solutions including raw T-SQL MERGE statements for zero-dependency scenarios and the Entity Framework Extensions BulkMerge API as a third-party alternative. It provides a clear comparison of when each approach is appropriate, helping developers choose the best strategy for their batch processing needs in .NET applications.