Why should you care?

Order validation with line items and conditional GST rules is unreadable with annotations alone.

See it live — copy this example

Create a Web API (dotnet new webapi), paste the example, run dotnet run, test in Swagger.

public class CreateProductValidator : AbstractValidator<CreateProductDto>
{
    public CreateProductValidator()
    {
        RuleFor(x => x.Name).NotEmpty().MaximumLength(120);
        RuleFor(x => x.Price).GreaterThan(0);
    }
}

Run Example »

Edit the code and click Run — like W3Schools Try it Yourself.

What happened?

• Follow the practice steps below on ShopNest.API — typing code yourself is the fastest way to learn.

Try it yourself

1. Install FluentValidation.AspNetCore.
2. Register validators in Program.cs.
3. POST invalid JSON and read 400 ProblemDetails.
4. Change a route URL or DTO property and save — test again in Swagger or curl.
5. Return the wrong status code on purpose (404 instead of 200) and see what the client shows.

Remember