Tutorials ASP.NET Core Web API Tutorial
400 HTTP Status Code in ASP.NET Core Web API — Complete Guide
400 HTTP Status Code in ASP.NET Core Web API — Complete Guide: free step-by-step lesson with examples, common mistakes, and interview tips — part of ASP.NET Core Web API Tutorial on Toolliyo Academy.
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ASP.NET Core Web API Tutorial · Lesson 45 of 175
400 HTTP Status Code in ASP.NET Core Web API
Beginner ✓ → Intermediate → Advanced → Professional
Intermediate · 2 — Data & pipeline · ~6 min · Module 4: Return Types and Status Codes
What is this?
400 Bad Request tells the client: invalid input. ASP.NET Core returns it with Ok(), Created(), NotFound(), StatusCode(400), and similar helpers.
Why should you care?
Mobile and SPA clients branch on status codes — not English messages. Wrong codes break retry logic and confuse Postman tests.
See it live — copy this example
Create a Web API (dotnet new webapi), paste the example, run dotnet run, test in Swagger.
return StatusCode(400); // Bad RequestRun Example »
This lesson uses terminal or setup steps. Run commands on your computer — the live editor appears on coding lessons.
What happened?
- Show validation errors to user
- Follow the practice steps below on ShopNest.API — typing code yourself is the fastest way to learn.
Try it yourself
- Trigger a 400 response in ShopNest.API (Swagger or Postman).
- Inspect status line and response body in the Network tab.
- Compare with the lesson on 404 — note the difference.
- Change a route URL or DTO property and save — test again in Swagger or curl.
- Return the wrong status code on purpose (404 instead of 200) and see what the client shows.
Remember
400 = Bad Request. Use the right helper method in ControllerBase. Clients depend on the number, not your message text.
Interview prep for this lesson
Practice these questions aloud after reading—each links to a full structured answer.
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