C# - email - prevent remember credentials

Question

I was doing tests to send an email from C #, for it use the following code:

        MailMessage message = new MailMessage();
        message.To.Add("user@gmail.com");
        message.From = new MailAddress("me@server.com");
        message.Subject = "Test subject";
        message.Body = "Test body";

        SmtpClient client = new SmtpClient("server");
        client.Timeout = 10000;
        client.EnableSsl = true;
        client.UseDefaultCredentials = false;

        NetworkCredential credentials = new NetworkCredential("me@server.com", "CorrectPassword");
        client.Credentials = credentials;
        try
        {
            client.Send(message); // OK
        }
        catch (Exception ex)
        {
            MessageBox.Show("Exception caught in CreateTestMessage2(): {0}",  ex.ToString());
        }

Then, try to start controlling the exceptions for connection interruption and mainly invalid key, but note that even using incorrect keys, the mail was still sent.

        NetworkCredential credentials = new NetworkCredential("me@server.com", "BadPassword");
        client.Credentials = credentials;
        try
        {
            client.Send(message); // OK, the email has been sent
        }
        catch (Exception ex)
        {
            MessageBox.Show("Exception caught in CreateTestMessage2(): {0}",  ex.ToString());
        }

How do I disable the automatic saving of credentials? Or, how do I erase the stored credentials?

What happens if you don't set the Credentials at all, does it still send? — Peter B, Aug 28 '18 at 21:06
Regarding this https://stackoverflow.com/a/27896967 , using `SmtpClient.UseDefaultCredentials = false` several times might help you. — isydmr, Aug 28 '18 at 21:19
@Angel - Please consider accepting the provided answer if it answers your question as it may help others in the future. — user1011627, Sep 07 '18 at 21:59

score 1 · Answer 1 · answered Aug 28 '18 at 22:07

The way that the SmtpClient works regarding those 2 properties (UseDefaultCredentials and Credentials) is dictated by the code MS has in place. Here is how they are implemented:

public bool UseDefaultCredentials {
            get {
                return (transport.Credentials is SystemNetworkCredential) ? true : false;
            }
            set {
                if (InCall) {
                    throw new InvalidOperationException(SR.GetString(SR.SmtpInvalidOperationDuringSend));
                }

                transport.Credentials = value ? CredentialCache.DefaultNetworkCredentials : null;
            }
        }


        public ICredentialsByHost Credentials {
            get {
                return transport.Credentials;
            }
            set {
                if (InCall) {
                    throw new InvalidOperationException(SR.GetString(SR.SmtpInvalidOperationDuringSend));
                }

                transport.Credentials = value;
            }
        }

This tripped me up a while back as I was setting the default to false after setting creds and I couldn't get gmail to authenticate. If you look deeper at the code around SmtpClient and SmtpTransport, the code appears to only make the connection a single time during the first send requests. Therefore, it doesn't appear that you can set the credentials "again" during the same client. If I am reading the code correctly, even calling UseDefaultCredentials again won't work AFTER the first send request. This code is what causes it to happen as far as I can tell:

private void GetConnection()
    {
        if (!_transport.IsConnected)
        {
            _transport.GetConnection(_host, _port);
        }
    }

Here are the links to the MS open source code if you want to dig around in there to see what it's doing.

https://source.dot.net/#System.Net.Mail/System/Net/Mail/SmtpClient.cs https://source.dot.net/#System.Net.Mail/System/Net/Mail/SmtpTransport.cs

C# - email - prevent remember credentials

1 Answers1