Cannot apply indexing with [] to an expression of type 'ADODB.Fields'

Code:

private void TestEmailSettings ()
        {
            CDO.Message message;
            message=new CDO.MessageClass();
            message.To=Settings.UserEmail;
            message.Subject=String.Format("{0} - Test message for {1}",Settings.ProductName,Settings.Name);
            message.HTMLBody="User: "+Settings.Name+"<BR>"+"This is a test Message";
            message.From=txtFrom.Text;

            CDO.Configuration config;
            config=new CDO.ConfigurationClass();
            config.Fields["http://schemas.microsoft.com/cdo/configuration/sendusing"].Value=2;   //ERROR
            config.Fields["http://schemas.microsoft.com/cdo/configuration/smtpserver"].Value=txtServer.Text;   //ERROR
            config.Fields["http://schemas.microsoft.com/cdo/configuration/sendusername"].Value=txtUser.Text; //ERROR
            config.Fields["http://schemas.microsoft.com/cdo/configuration/sendpassword"].Value=txtPassword.Text;  //ERROR
            config.Fields["http://schemas.microsoft.com/cdo/configuration/smtpauthenticate"].Value=1;  //ERROR
            config.Fields["http://schemas.microsoft.com/cdo/configuration/smtpserverport"].Value=Convert.ToInt32(txtPort.Text);   //ERROR
            config.Fields["http://schemas.microsoft.com/cdo/configuration/smtpusessl"].Value=chkUseSSL.Checked;  //ERROR
            
            config.Fields.Update();
            message.Configuration=config;

            message.Send();
        }

Code:

config.Fields["http://schemas.microsoft.com/cdo/configuration/sendusing"].Value=2;