ASP.NET Core: Why I Couldn’t Upgrade FluentValidation Past 11.4 in My Calzolari.Grpc.AspNetCore.Validation Package (and How I Finally Fixed It)

This article details the author's experience maintaining the Calzolari.Grpc.AspNetCore.Validation package, which was stuck on FluentValidation 11.4.0 due to a runtime crash when upgrading. The root cause is a change in FluentValidation 11.5+ that made the IValidator<T> interface contravariant (adding 'in' keyword), causing reflection-based assembly scanning to fail at startup. The article provides a technical deep dive into the issue, including code examples and the eventual fix. It is relevant to IT/TECHNOLOGY as it covers ASP.NET Core, gRPC, FluentValidation, dependency injection, and runtime debugging.