C# Cannot resolve symbol

Question

I don't know why I'm getting this error. Seems elementary. Anyway I have a singleton class called EmailSender. The code below is short and easy. The issue is that I can't use sender in the MainWindow class. Anything I try such as sender.Send() is treated as though I've done asdafsafas.Send(). It's treated as though it's a random string of characters. Don't know why this is happening.

using System;
using System.Net.Mail;
using System.Windows.Forms;

namespace SendMail
{
    public partial class MainWindow : Form
    {
        #region Private variables
        private MailMessage msg = new MailMessage();
        private EmailSender sender = EmailSender.GetInstance();
        #endregion

        public MainWindow()
        {
            InitializeComponent();

        }

        private MailMessage PrepareMailMessage()
        {

            return msg;
        }

        private void btnSend_Click(object sender, EventArgs e)
        {

        }
    }
}

Here is the GetInstance method:

public static EmailSender GetInstance()
{
    return _instance ?? (_instance = new EmailSender());
}

Answer 1

This is because of the way you have this method defined (sender is a parameter). It's finding the method argument first , not your class level variable. You can qualify this:

private void btnSend_Click(object sender, EventArgs e)
{
    // sender here is the "(object sender, " paramater, so it's defined
    // as system object.

    // use this instead:
    this.sender.Send(); // The "this" will make the class find the instance level variable instead of using the "object sender" argument
}

Answer 2

That's because sender is not a mail object, but rather the button that triggered the event. You need the SmtpClient to send the email:

private void btnSend_Click(object sender, EventArgs e)  
{  
    SmtpClient client = new SmtpClient("192.0.0.1", 25); //host, port
    client.Send(msg);
}

Also, the MailMessage class implements IDisposable , so you need some code to dispose of the message once you're done with it.

I created a wrapper that includes everything you need to send emails, including disposal:

/// <summary>
/// Wrapper class for the System.Net.Mail objects
/// </summary>
public class SmtpMailMessage : IDisposable
{
    #region declarations

    MailMessage Message;
    SmtpClient SmtpMailClient;

    #endregion

    #region constructors

    /// <summary>
    /// Default constructor for the SmtpMailMessage class
    /// </summary>
    public SmtpMailMessage()
    {
        //initialize the mail message
        Message = new MailMessage();
        Message.Priority = MailPriority.Normal;
        Message.DeliveryNotificationOptions = DeliveryNotificationOptions.OnFailure;            
        Message.From = new MailAddress("xxx@abc.com");           

        //initialize the smtp client
        SmtpMailClient = new SmtpClient();
        SmtpMailClient.DeliveryMethod = SmtpDeliveryMethod.Network;
        SmtpMailClient.Host = "192.168.0.1";
        SmtpMailClient.Port = 25;
    }

    /// <summary>
    /// Parameterized constructor for the SmtpMailMessage class. Allows for override of the default
    /// SMTP host and port number
    /// </summary>
    /// <param name="HostIP">The IP address of the exchange server</param>
    /// <param name="PortNumber">The port number for ingoing and outgoing SMTP messages</param>
    public SmtpMailMessage(string HostIP, int PortNumber) : this()
    {
        //override the smtp host value
        SmtpMailClient.Host = HostIP;

        //override the smtp port value
        SmtpMailClient.Port = PortNumber;
    }

    #endregion

    #region subject / body

    /// <summary>
    /// The body content of the mail message
    /// </summary>
    public string Body
    {
        get
        {
            return Message.Body;
        }
        set
        {
            Message.Body = value;
        }
    }

    /// <summary>
    /// the subject of the mail message
    /// </summary>
    public string Subject
    {
        get
        {
            return Message.Subject;
        }
        set
        {
            Message.Subject = value;
        }
    }

    #endregion

    #region mail type

    /// <summary>
    /// Gets or sets a value that determines whether the mail message
    /// should be formatted as HTML or text
    /// </summary>
    public bool IsHtmlMessage
    {
        get
        {
            return Message.IsBodyHtml;
        }
        set
        {
            Message.IsBodyHtml = value;
        }
    }

    #endregion

    #region sender

    /// <summary>
    /// Gets or sets the from address of this message
    /// </summary>
    public string From
    {
        get
        {
            return Message.From.Address;
        }
        set
        {
            Message.From = new MailAddress(value);
        }
    }

    #endregion

    #region recipients

    /// <summary>
    /// Gets the collection of recipients
    /// </summary>
    public MailAddressCollection To
    {
        get
        {
            return Message.To;

        }
    }

    /// <summary>
    /// Gets the collection of CC recipients 
    /// </summary>
    public MailAddressCollection CC
    {
        get
        {
            return Message.CC;
        }
    }

    /// <summary>
    /// Gets the collection of Bcc recipients
    /// </summary>
    public MailAddressCollection Bcc
    {
        get
        {
            return Message.Bcc;
        }
    }

    #endregion

    #region delivery notification

    /// <summary>
    /// Gets or sets the delivery notification settings for this message
    /// </summary>
    public DeliveryNotificationOptions DeliveryNotifications
    {
        get
        {
            return Message.DeliveryNotificationOptions;
        }
        set
        {
            Message.DeliveryNotificationOptions = value;
        }
    }

    #endregion

    #region priority

    /// <summary>
    /// Gets or sets the Priority of this message
    /// </summary>
    public MailPriority PriorityLevel
    {
        get
        {
            return Message.Priority;
        }
        set
        {
            Message.Priority = value;
        }
    }

    #endregion

    #region send methods

    /// <summary>
    /// Sends the message anonymously (without credentials)
    /// </summary>
    public void Send()
    {
        SmtpMailClient.Send(Message);
    }

    /// <summary>
    /// Sends the message with authorization from a network account   
    /// </summary>
    /// <param name="Username">The Windows username of the authorizing user</param>
    /// <param name="Password">The Windows password of the authorizing user</param>
    /// <param name="Domain">The domain name of the network to which the authorizing user belongs</param>
    public void Send(string Username, string Password, string Domain)
    {
        //attach a network credential to this message using the information passed into the method
        SmtpMailClient.Credentials = new NetworkCredential(Username, Password, Domain);

        //send the message
        SmtpMailClient.Send(Message);
    }

    #endregion

    #region IDisposable implementation

    ~SmtpMailMessage()
    {
        Dispose(false);
    }

    public void Dispose()
    {
        Dispose(true);            
    }

    protected virtual void Dispose(bool disposing)
    {
        if (disposing)
        {
            if (Message != null)
                Message.Dispose();
            Message = null;                
            SmtpMailClient = null;
        }
    }

    #endregion        
}

Implementation:

using (SmtpMailMessage mail = new SmtpMailMessage("192.168.0.1", 25))
{
    //set the to address to the primary email
    mail.To.Add("xxx@abc.com");

    //set the message type and subject and body
    mail.IsHtmlMessage = true;
    mail.Subject = "Foo";
    mail.Body = "Hello world!";

    //send the email
    mail.Send();
}

Answer 3

If you're doing it inside the scope of your btnSend_Click method, the parameter object sender takes precedence over the global EmailSender sender .

You should either rename your global variable, eg: EmailSender m_sender or specify exactly which sender you want: this.sender.Send()

Answer 4

I guess you call the sender.Send in the btnSend_Click function.

In that function is a param also called sender (object sender). Now you code is confused which one to use. So rename your private var sender.