Backend receives null GUID instead of actual value

Backend
1

I’m working with a C# Web API and React TypeScript frontend. When I send a GUID from my frontend to the backend endpoint, the backend always gets an empty GUID (all zeros) instead of the actual value I’m sending.

Backend Controller:

[Route("FetchByUser")]
[HttpGet("{userId}")]
public async Task<ActionResult<List<Appointment>>> FetchByUser(Guid userId)
{
    Console.WriteLine(userId); // Shows: 00000000-0000-0000-0000-000000000000
    // rest of logic here
}

Frontend Service Function:

export const fetchAppointmentsByUser = async (userId: string) => {
    const endpoint = "http://localhost:5032/appointments/FetchByUser?userId=" + userId;
    console.log(endpoint); // URL looks correct in console
    // API call logic
}

Component Usage:

useEffect(() => {
    setAppointments([]);
    const loadAppointments = async () => {
        const testUserId = "A1B2C3D4-E5F6-7890-ABCD-EF1234567890";
        const result = await fetchAppointmentsByUser(testUserId);
        setAppointments(result);
    }
    loadAppointments();
}, []);

The URL in the browser console shows the correct GUID, but the backend parameter is always empty. No errors are thrown. What could be causing this issue?

2

yeah, exactly what growingtree said. your route template has {userId} so it’s looking for the guid in the path like /FetchByUser/A1B2C3D4-E5F6-7890-ABCD-EF1234567890. you’re sending it as a query param with ?userId= instead. that’s why it’s coming back null - the route isn’t matching.

3

You’ve got a mismatch between your route and how you’re calling it. Your controller has [HttpGet("{userId}")] which expects the GUID in the URL path, but your frontend is sending it as a query string parameter instead. That’s why the backend gets an empty GUID - the route parameter userId isn’t getting populated from the path. Change your frontend to "http://localhost:5032/appointments/FetchByUser/" + userId so the GUID goes in the URL path, not as a query parameter. This’ll match your route template and properly populate the userId parameter.

4

wait, are you mixing route parameters and query parameters? your controller expects {userId} in the route but your frontend sends it as ?userId=. shouldn’t your endpoint just be /FetchByUser/ + userId? what happens if you try that?