Xamarin.Forms中Android平台WebRequest数据获取失败求助

Your code works perfectly on Windows, but Android has a few unique network-related quirks that are likely causing this problem. Let’s walk through the most common fixes step by step:

1. Double-check Android network permissions #

Android requires explicit permission to access the internet. Make sure your AndroidManifest.xml includes these lines (you can also enable them via Visual Studio’s Android Manifest project settings):

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" /> <!-- Optional but helpful for debugging -->

2. Switch to AndroidClientHandler for better compatibility #

Xamarin.Android’s default HttpClient handler can have compatibility issues on some devices. Try updating your HttpClient initialization:

private HttpClient _Client = new HttpClient(new AndroidClientHandler());

Also, go to your Android project’s Properties > Android Options > Advanced and set "HttpClient Implementation" to Android (instead of Default).

3. Force TLS 1.2 support (for older Android devices) #

Devices running Android 7.0 or below don’t enable TLS 1.2 by default, and jsonplaceholder.typicode.com uses this secure protocol. Add this code to your MainActivity.cs’s OnCreate method:

protected override void OnCreate(Bundle savedInstanceState)
{
    // Enable TLS 1.2 for secure connections
    ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
    base.OnCreate(savedInstanceState);
    // Rest of your OnCreate code...
}

4. Fix the async void lifecycle issue (and get better error details) #

Using async void in OnAppearing can swallow exceptions or cause lifecycle conflicts. Refactor your code to use a separate async method, and print the full exception chain to see the real root cause:

protected override async void OnAppearing()
{
    await LoadPostsAsync();
    base.OnAppearing();
}

private async Task LoadPostsAsync()
{
    try
    {
        var content = await _Client.GetStringAsync(url);
        var post = JsonConvert.DeserializeObject<List<Post>>(content);
        _post = new ObservableCollection<Post>(post);
        Post_List.ItemsSource = _post;
        await DisplayAlert("content", content, "ok");
    }
    catch (Exception ex)
    {
        // Print the full exception stack to see inner errors
        var currentEx = ex;
        while (currentEx != null)
        {
            Debug.WriteLine($"Error: {currentEx.Message}");
            currentEx = currentEx.InnerException;
        }
    }
}

This will show you exactly what’s going on—whether it’s a certificate issue, network timeout, or something else.

5. Handle Android 9+ cleartext traffic (if needed) #

While you’re using HTTPS, if your URL ever redirects to HTTP (unlikely here, but worth checking), Android 9+ blocks plaintext traffic by default. Add this to your AndroidManifest.xml’s application tag:

android:usesCleartextTraffic="true"

Start with checking permissions and printing the full exception—those are usually the fastest ways to pinpoint the problem.

内容的提问来源于stack exchange，提问作者Henrique Marques