Building HTTP Web APIs That Other Apps Call
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When two people edit the same row, the second save overwrites the first, and nobody finds out. A concurrency token fixes this: a column that changes with every update, and that EF Core adds to the WHERE of each UPDATE. If it changed since you read the row, the update finds nothing, and you can tell the client. This guide adds one on MySQL.
Clients want to choose a list's sort order, as in ?sort=-price. It's tempting to pass that text through to the query, but a query string should never decide which columns your database sorts on. An allow-list and a C# switch give clients the choice, and keep the control in your code.
A list endpoint usually has optional filters: a category, a search term, a checkbox. Since an EF Core query doesn't run until the end, you can add each Where only when its filter is present, and get SQL with exactly those conditions. This guide builds that, and shows the SQL for different requests.
A list endpoint shouldn't return every row. Skip and Take give you one page at a time, which EF Core turns into MySQL's LIMIT and OFFSET. Getting it right takes three more things: a sort order rows can't escape from, the total count, and a limit on the page size. This guide builds all of it.
To delete a row with EF Core, load it, call Remove, and call SaveChangesAsync. A Web API also needs to answer the two things that can go wrong: the row isn't there, and other rows still depend on it. This guide does all of that, with the real SQL from MySQL.
ASP.NET Core can describe your API as an OpenAPI document, but that's a JSON file meant for tools. Scalar turns it into a page you can read and try in a browser. It takes one package and one line of code. This guide adds it to a .NET 10 C# Web API, for Development only.
A .http file is a plain text file that holds API requests. It lives next to your code and goes into Git with it, so everyone on the team has the same requests. VS Code with the free REST Client extension and Visual Studio both add a link above each request that sends it. This guide covers the format and how to send requests in each editor.
After I added one small GET endpoint, every request to my minimal API answered 500, health checks included. The log said Body was inferred but the method does not allow inferred body parameters. The endpoint took a record as a parameter, and ASP.NET Core assumes that comes from the JSON body, which GET doesn't get. [AsParameters] tells it to read the query string instead.
A request with no price was accepted, and the product's price quietly became 0. No error, no warning. A C# decimal can't be null, so when the JSON leaves it out, it simply keeps its default value, and [Required] has nothing to complain about. Making the property decimal? fixes that.
A POST with perfectly good JSON came back as 415 Unsupported Media Type, and the API's log had nothing to say about it. The JSON wasn't the problem: the request never said it was JSON. curl labels a -d body as a web form unless you tell it otherwise, and a minimal API endpoint that expects JSON won't touch that.
