Fixing AAD Authentication and SSL Issues in IIS — Why We Migrated from System.Data.SqlClient to Microsoft.Data.SqlClient

"Everything was working ****fine. Then someone added an Azure database connection string. And suddenly, nothing was fine." — Every senior developer on a legacy project, at least once 🧭 Overview I have been working on enterprise .NET applications for over 11+ years now. Most of them are what people politely call "legacy systems" — Web Forms,API's, IIS deployments, App.config files, the works. These applications run without issues for years. On-prem SQL Server connections are rock solid. Everyone is happy. Then one day, your team decides to add a cloud database — Azure SQL — because the client wants to "move to the cloud" or some new feature needs it. You add a new connection string, you test locally, it kind of works, and then you deploy to IIS. That is when the fun begins. This article is about exactly that situation — what went wrong in one of our production systems, why it went wrong, and how we fixed it properly. If you are working on a legacy .NET app and adding Azure SQL connectio