Hello Can I learn how to fix this problem ?

Hello Hakan Arlı ,

From your screenshot, the controller is calling HttpClient, that call returns 404, and then your server throws when EnsureSuccessStatusCode runs, so the browser shows 500. In short: the real problem is a bad URL or route mismatch in the HttpClient request, and your API is not handling that failure.

Try this quick checklist:

• Verify the real route
  • Hit the API directly in the browser/Postman: https://localhost:7156/projects.
  • If it’s 404 there, check your controller routing. For attribute routes, make sure they line up, e.g.:
    • [Route("api/[controller]")] + [HttpGet] → /api/projects
    • [ApiController][Route("[controller]")] + [HttpGet("projects")] → /projects
  • Ensure app.MapControllers() (or minimal API mappings) are in Program.cs.
• Fix the HttpClient URL construction
  • If BaseAddress is set, you should pass only the relative path (no string interpolation with the base), e.g.:

        var client = _httpClientFactory.CreateClient("Api"); // has BaseAddress = https://localhost:7156
        var response = await client.GetAsync("projects"); // not $"{BaseAddress}/projects"
    

  • Double‑check port and scheme (http vs https) to match what Kestrel actually listens on.
• Don’t convert a 404 into a 500
  • Avoid EnsureSuccessStatusCode() unless you really want to throw. Handle not‑found explicitly and return something appropriate:

        var client = _httpClientFactory.CreateClient("Api");
        var response = await client.GetAsync("projects");
     
        if (response.StatusCode == HttpStatusCode.NotFound)
            return NotFound(); // or an empty list, as you prefer
     
        if (!response.IsSuccessStatusCode)
            return StatusCode((int)response.StatusCode, await response.Content.ReadAsStringAsync());
     
        var projects = await response.Content.ReadFromJsonAsync<List<ProjectDto>>();
        return Ok(projects);
    

  • If you prefer one‑liner deserialization, avoid the throw:

        var projects = await response.Content.ReadFromJsonAsync<List<ProjectDto>>();
    

• Add temporary diagnostics
  • Log the final request URI and status code:

        _logger.LogInformation("Requesting {Uri}", response.RequestMessage!.RequestUri);
        _logger.LogInformation("Status {Code}", (int)response.StatusCode);
    

  • In Program.cs, enable detailed errors for local dev if needed.
• Common routing pitfalls to check
  • Mixed routes where Swagger shows /api/projects but your client calls /projects.
  • Missing [ApiController] or route attributes on the controller/action.
  • Extra path base or reverse proxy prefix not accounted for.
  • CORS is not your issue here; a 404 reached your server.

If you share the controller attributes and the HttpClient setup (named client config and the line that builds the URL), it’ll be straightforward to pinpoint the exact mismatch.

Hope this helps you resolve it quickly!

A 4XX status code means there's a client error. In this case, the 404 (Not Found) error suggests the URL you used with HttpClient doesn't exist. To fix this, you need to check if the URL you're calling is the one you're passing to the HttpClient.

A 5XX status code is a server-side error. The 500 error you're getting is likely happening because the server isn't properly handling the 404 (Not Found) response it received from the HttpClient call.

If you need more help, please share your code so the community can debug it.

The root cause of issue is HTTP 404. It means Not Found which means that name resolution has been properly complete, request from client has reached to web server, web server has tried to find the resource specified by url but could not find the resource at the url.

The cause is most likely wrong url which includes simple typo, wrong use of relative path, wrong routing in the server, use of wrong folder, wrong server requested and others.

Even if the url is correct, the server may return HTTP 404 during the process of request according to your programming of web app (e.g., web app tried to find the requested record in database but there is no requested record).

Only you will be able to find the cause. Therefore, first please examine the above mentioned causes by yourself.