Building HTTP Web APIs That Other Apps Call
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You save DateTime.UtcNow with EF Core, read it back from MySQL, and the Z that marks UTC is gone from your API's JSON. Worse, a DateTime.Now slipped in somewhere is stored as local time, hours away from everything else. This guide shows both problems with a real .NET C# Web API and MySQL 9.7, explains why they happen, and fixes them for every DateTime in the model with one small value converter.
System.IO.IOException: Failed to bind to address http://127.0.0.1:5190: address already in use. The app is fine; the port isn't free. Something else, very often another copy of the same app, is already listening on it. Here's how I found what held the port, stopped it cleanly, and when to pick another port instead.
Four different mistakes, one message: Microsoft.EntityFrameworkCore.DbUpdateException: Could not save changes. Please configure your entity type accordingly. A duplicate name, a negative price, a missing category and a blocked delete all produced it, through Oracle's MySQL provider for EF Core. Nothing was wrong with any entity type's configuration. The real reason sits one line lower, in the inner MySqlException, and its error number is what your code should handle.
A Web API needs somewhere to keep its data, and MySQL is one of the most popular places to keep it. This part connects an ASP.NET Core minimal API, written in .NET 10 and C#, to MySQL 9.7 through Entity Framework Core. It keeps the password out of the code, encrypts the connection, proves the connection works with a health check, and then looks at that same connection from inside MySQL so you can see exactly what happened.
A health check is a small endpoint that answers one question: is this API working? Hosting platforms, load balancers and monitoring tools call it every few seconds and act on the answer. This guide adds two of them to a .NET 10 C# minimal API, one that only checks that the API is running and one that also checks the database, and shows their real answers when the database goes down.
A real .NET C# Web API usually isn't one project. It's a solution: the API itself, plus one or more class libraries for code that doesn't belong in the web layer, such as the database code. This guide builds that structure from the command line with .NET 10, adds the settings that production projects use, and fixes the two errors those settings raise on the template's own code.
