[TestMethod]
        public async Task RemoteCertificate_ForceFailure()
        {
            // Verifies that the default certificate validator is called for a normal request, and that it can be forced to fail
            int calls = 0;
            Exception callbackAssertFail = null;

#if __IOS__
            var handler = new NSUrlSessionHandler();
            handler.ServerCertificateCustomValidationCallback = (sender, certificate, chain, sslPolicyErrors) =>
#else
            var handler = new System.Net.Http.SocketsHttpHandler();
            handler.SslOptions.RemoteCertificateValidationCallback = (sender, certificate, chain, sslPolicyErrors) =>
#endif
            {
                Interlocked.Increment(ref calls);
                try
                {
                    Assert.AreEqual(SslPolicyErrors.None, sslPolicyErrors, "sslPolicyErrors"); // Fails on iOS since 9.0.300. Now returns `RemoteCertificateChainErrors`
                }
                catch (Exception ex)
                {
                    callbackAssertFail = ex;
                }
                return false;
            };

            var client = new HttpClient(handler);
            // Disable keep-alive and cache to force reconnection for each request
            client.DefaultRequestHeaders.ConnectionClose = true;
            client.DefaultRequestHeaders.CacheControl = new CacheControlHeaderValue { NoCache = true, NoStore = true };

            var responseTask = client.GetAsync(new Uri("https://sha256.badssl.com/")); // valid certificate
            try { await responseTask; }
            catch (HttpRequestException) { /* expected */ }
            
            // Expect our validator to be called once
            Assert.AreEqual(1, calls, "numCalls");
            if (callbackAssertFail != null)
            {
                throw callbackAssertFail;
            }
        }