System.Net.Http.WinHttpRequestCallback.OnRequestSendingRequest C# (CSharp) Method

        private static void OnRequestSendingRequest(WinHttpRequestState state)
        {
            Debug.Assert(state != null, "OnRequestSendingRequest: state is null");
            Debug.Assert(state.RequestHandle != null, "OnRequestSendingRequest: state.RequestHandle is null");
            
            if (state.RequestMessage.RequestUri.Scheme != UriScheme.Https)
            {
                // Not SSL/TLS.
                return;
            }

            // Grab the channel binding token (CBT) information from the request handle and put it into
            // the TransportContext object.
            state.TransportContext.SetChannelBinding(state.RequestHandle);

            if (state.ServerCertificateValidationCallback != null)
            {
                IntPtr certHandle = IntPtr.Zero;
                uint certHandleSize = (uint)IntPtr.Size;

                if (!Interop.WinHttp.WinHttpQueryOption(
                    state.RequestHandle,
                    Interop.WinHttp.WINHTTP_OPTION_SERVER_CERT_CONTEXT,
                    ref certHandle,
                    ref certHandleSize))
                {
                    int lastError = Marshal.GetLastWin32Error();
                    WinHttpTraceHelper.Trace(
                        "OnRequestSendingRequest: Error getting WINHTTP_OPTION_SERVER_CERT_CONTEXT, {0}",
                        lastError);

                    if (lastError == Interop.WinHttp.ERROR_WINHTTP_INCORRECT_HANDLE_STATE)
                    {
                        // Not yet an SSL/TLS connection. This occurs while connecting thru a proxy where the
                        // CONNECT verb hasn't yet been processed due to the proxy requiring authentication.
                        // We need to ignore this notification. Another notification will be sent once the final
                        // connection thru the proxy is completed.
                        return;
                    }

                    throw WinHttpException.CreateExceptionUsingError(lastError);
                }
                
                // Create a managed wrapper around the certificate handle. Since this results in duplicating
                // the handle, we will close the original handle after creating the wrapper.
                var serverCertificate = new X509Certificate2(certHandle);
                Interop.Crypt32.CertFreeCertificateContext(certHandle);

                X509Chain chain = null;
                SslPolicyErrors sslPolicyErrors;

                try
                {
                    WinHttpCertificateHelper.BuildChain(
                        serverCertificate,
                        state.RequestMessage.RequestUri.Host,
                        state.CheckCertificateRevocationList,
                        out chain,
                        out sslPolicyErrors);

                    bool result = state.ServerCertificateValidationCallback(
                        state.RequestMessage,
                        serverCertificate,
                        chain,
                        sslPolicyErrors);
                    if (!result)
                    {
                        throw WinHttpException.CreateExceptionUsingError(
                            (int)Interop.WinHttp.ERROR_WINHTTP_SECURE_FAILURE);
                    }
                }
                finally
                {
                    if (chain != null)
                    {
                        chain.Dispose();
                    }

                    serverCertificate.Dispose();
                }
            }
        }