Removing Delegates

Removing delegates is the opposite of combining them.

To remove a delegate from a combined delegate:

1. To remove a delegate, you must have two variables : one that stores a combination of delegates and one that points to the delegate object you want to remove from the list.

2. Type the name of the variable that has the combined delegate.

3. Type -= (minus equal operator).

4. Type the name of the variable you want to remove from the list of delegates.

5. Type a semicolon ; ( Figure 10.9 ).

   Figure 10.9 With the -= operator we can remove one of the delegates from the combined delegate. Therefore, calling t3 now only invokes Task2.

    class Tasks {    public bool Task1(string desc)    {       return true;    }    public static bool Task2(string desc)    {       return true;    } } delegate bool TaskDel(string desc); public class WebForm1 : System.Web.UI.Page {    private void Page_Load(object sender,                 System.EventArgs e)    {       Tasks tsks = new Tasks();       TaskDel t1 = new TaskDel(                       tsks.Task1);       TaskDel t2 = new TaskDel(                       Tasks.Task2);       bool result;       result = t1("Calling Task 1...");       result = t2("Calling Task 2...");       TaskDel t3 = t1 + t2;       t3("Call both delegates...");  t3 -= t1;   t3("Call only Task2...");  } } 

Tips

• When you use the -= operator, the C# compiler turns that code into a call to the delegate's Remove function, which lets you remove a delegate from a linked list of delegates.

• The ability of delegates to be combined and removed makes it easy to build a subscriber class such as the Modem class ( Figure 10.10 ). This class reports information for the status of the modem, and multiple classes can subscribe to receive notifications. The Modem class has a method called Subscribe and a method called Unsubscribe to enable other classes to request and cancel notifications.

  Figure 10.10 The Modem class enables a user to request modem status as the modem is connecting. The Modem class has a Subscribe method that enables a user to pass a delegate variable. The Connect method then uses this variable to call a function on the subscriber's class.

   delegate void ReportStatusDel(               string status);  class Modem  {    static ReportStatusDel report;    public static void  Subscribe  (                 ReportStatusDel del)    {  report += del;  }    public static void  Unsubscribe  (                 ReportStatusDel del)    {  report -= del;  }    public static void Connect()    {       if (report != null)       {  report("Initializing...");   report("Dialing...");   report("Authenticating...");   report("Connected...");  }    } } public class WebForm1 : System.Web.UI.Page {    private void  ReportModemStatus  (                string msg)    {       Response.Write(msg + "<br>");    }    private void Page_Load(object sender,                 System.EventArgs e)    {  Modem.Subscribe(   new ReportStatusDel(   ReportModemStatus));   Modem.Connect();   Modem.Unsubscribe(   new ReportStatusDel(   ReportModemStatus));  } }