httpClientHandler.ServerCertificateCustomValidationCallback never executed

Question

I'm using Xamarin, in VS 2022, class libraries targeting .NetStanderd2.1, for Android.

I'd like to implement an HttpCient with custom SSL certificate validation.

Here's my code:

var httpClientHandler = new HttpClientHandler();
httpClientHandler.ServerCertificateCustomValidationCallback +=
    (sender, cert, chain, sslPolicyErrors) =>
    {
        return true;
    };
var httpClient = new HttpClient(httpClientHandler);
httpClient.BaseAddress = new Uri(baseUrl);
httpClient.Timeout = TimeSpan.FromSeconds(10);
HttpResponseMessage response =
    await httpClient.GetAsync("api/status");
response.EnsureSuccessStatusCode();

(somewhat prepared for legibility).

I switched the Xamarin HttpClient to Managed

I understand the security implications of disabling SSL certificate validation, this is in a development environment, using a self-signed certificate.

If possible, I'd like to use TLS 1.2, but that problem is next.

The current problem is, that the custom SSL cert verification (return true;) in

httpClientHandler.ServerCertificateCustomValidationCallback +=
    (sender, cert, chain, sslPolicyErrors) =>
    {
        return true;
    };

is never executed.

What am I doing wrong?

Answer 1

This should be a known issue, you can check it here: https://github.com/xamarin/xamarin-android/issues/4688 .

But you can try to resolve it by changing the implementation of HTTP handler from xamarin to Android.

Please refer to the following code:

In form, create interface INativeHttpMessageHandlerProvider :

public interface INativeHttpMessageHandlerProvider
{
    HttpMessageHandler Get();
}

and create class HttpClientProvider

public class HttpClientProvider
{

        public HttpClient Get()
        {
            var nativeHttpMessageHandler = DependencyService.Get<INativeHttpMessageHandlerProvider>().Get();
            return new HttpClient(nativeHttpMessageHandler);   
            // nativeHttpMessageHander is injected from Android project
        }
    }

In android,implement interface INativeHttpMessageHandlerProvider

[assembly: Dependency(typeof(AndroidHttpMessageHandlerProvider))]
namespace SmartConnect.Droid.PlatformSpecific
{
    public class AndroidHttpMessageHandlerProvider : INativeHttpMessageHandlerProvider
    {
        public HttpMessageHandler Get()
        {
            return new AndroidClientHandler();
        }
    }
}

Refer : https://github.com/xamarin/xamarin-android/issues/4688#issuecomment-658833938