Web API & HttpClient

Imports System.Net.Http
Imports System.Text.Json

' ✓ CORRECT — single instance, module-level
Private Shared _http As New HttpClient() With {
    .BaseAddress = New Uri("https://api.openweathermap.org/data/2.5/"),
    .Timeout     = TimeSpan.FromSeconds(10)
}

' Add default headers once
Shared Sub New()
    _http.DefaultRequestHeaders.Add("Accept", "application/json")
    _http.DefaultRequestHeaders.Add("User-Agent", "VBStudentApp/1.0")
End Sub

' ✗ WRONG — creates a new socket on every call (socket exhaustion)
Private Async Sub btnLoad_Click(...)
    Using http As New HttpClient()   ' never do this!
        ...
    End Using
End Sub

' --- Response class matching the API JSON structure ---
Public Class WeatherResponse
    <JsonPropertyName("name")>
    Public Property CityName As String
    <JsonPropertyName("main")>
    Public Property Main As WeatherMain
    <JsonPropertyName("weather")>
    Public Property Weather As WeatherCondition()
End Class
Public Class WeatherMain
    <JsonPropertyName("temp")>     Public Property Temp       As Double
    <JsonPropertyName("humidity")> Public Property Humidity   As Integer
End Class
Public Class WeatherCondition
    <JsonPropertyName("description")> Public Property Description As String
End Class

' --- Fetch and display ---
Private Async Sub btnGetWeather_Click(...) Handles btnGetWeather.Click
    Dim city = txtCity.Text.Trim()
    If city = "" Then Return
    btnGetWeather.Enabled = False
    Try
        Dim url  = $"weather?q={Uri.EscapeDataString(city)}&appid={API_KEY}&units=metric"
        Dim json = Await _http.GetStringAsync(url)
        Dim data = JsonSerializer.Deserialize(Of WeatherResponse)(json)
        lblCity.Text   = data.CityName
        lblTemp.Text   = $"{data.Main.Temp:F1} °C"
        lblHumid.Text  = $"{data.Main.Humidity}% humidity"
        lblDesc.Text   = data.Weather(0).Description
    Catch ex As HttpRequestException
        MessageBox.Show($"Network error: {ex.Message}")
    Catch ex As TaskCanceledException
        MessageBox.Show("Request timed out.")
    Catch ex As JsonException
        MessageBox.Show($"Unexpected response format: {ex.Message}")
    Finally
        btnGetWeather.Enabled = True
    End Try
End Sub

' --- POST: send JSON body ---
Private Async Sub PostStudentAsync(s As Student)
    Try
        Dim response = Await _http.PostAsJsonAsync("api/students", s)
        response.EnsureSuccessStatusCode()   ' throws on 4xx/5xx
        Dim created = JsonSerializer.Deserialize(Of Student)(
            Await response.Content.ReadAsStringAsync())
        lblStatus.Text = $"Created ID={created.StudentID}"
    Catch ex As HttpRequestException When ex.StatusCode = Net.HttpStatusCode.Conflict
        lblStatus.Text = "Duplicate — student already exists (409)"
    Catch ex As HttpRequestException When ex.StatusCode = Net.HttpStatusCode.Unauthorized
        lblStatus.Text = "Auth failed — check API key (401)"
    Catch ex As HttpRequestException
        lblStatus.Text = $"HTTP error {ex.StatusCode}: {ex.Message}"
    End Try
End Sub

' --- Manual status check (without EnsureSuccessStatusCode) ---
Dim response2 = Await _http.GetAsync("api/students/999")
Select Case response2.StatusCode
    Case Net.HttpStatusCode.OK
        Dim json = Await response2.Content.ReadAsStringAsync()
        ' deserialise…
    Case Net.HttpStatusCode.NotFound
        MessageBox.Show("Student 999 not found.")
    Case Net.HttpStatusCode.TooManyRequests
        MessageBox.Show("Rate limit exceeded — wait before retrying.")
    Case Else
        MessageBox.Show($"Unexpected: {response2.StatusCode}")
End Select