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  4. delegates and events

delegates and events

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  • S staticv

    // Holds the arguments for the StatusChanged event

    public class StatusChangedEventArgs : EventArgs

    {

    // The argument we're interested in is a message describing the event

private string EventMsg;



// Property for retrieving and setting the event message

public string EventMessage

{

    get

    {

        return EventMsg;

    }

    set

    {

        EventMsg = value;

    }

}



// Constructor for setting the event message

public StatusChangedEventArgs(string strEventMsg)

{

    EventMsg = strEventMsg;

}

    }

    // This delegate is needed to specify the parameters we're passing with our event
    public delegate void StatusChangedEventHandler(object sender, StatusChangedEventArgs e);

    public static event StatusChangedEventHandler StatusChanged; Okay, so in the above code, we created a delegate and a custom event.

    // This is called when we want to raise the StatusChanged event

public static void OnStatusChanged(StatusChangedEventArgs e)

{

    **StatusChangedEventHandler statusHandler = StatusChanged;**

    if (statusHandler != null)

    {

        // Invoke the delegate

        statusHandler(null, e);

    }

}

    What does the bold line do? Why do we check for null? EDIT: The code is from C# Chat: Part 2 - Building the Chat Server (C# Programming Tutorial) • Geekpedia[^] check that out, if the above code doesn't provides what I mean.

    Top Web Hosting Providers[^] Do, or do not. There is no 'try'.

    L Offline
    L Offline
    Luc Pattyn
    wrote on last edited by
    #2

    Hi, you make a local copy of StatusChanged, so the same value gets used both in the null-text and the invocation; StatusChanged is public, so it could be changed by another thread at any point in time; removing the last handler from it right after the null-test got executed would cause an exception in statusHandler(null, e); In summary, without it your code would not be thread-safe. :)

    Luc Pattyn [Forum Guidelines] [My Articles]

    - before you ask a question here, search CodeProject, then Google - the quality and detail of your question reflects on the effectiveness of the help you are likely to get - use the code block button (PRE tags) to preserve formatting when showing multi-line code snippets

    modified on Friday, June 10, 2011 12:01 PM

    S J 2 Replies Last reply
    0
    • S staticv

      // Holds the arguments for the StatusChanged event

      public class StatusChangedEventArgs : EventArgs

      {

      // The argument we're interested in is a message describing the event

private string EventMsg;



// Property for retrieving and setting the event message

public string EventMessage

{

    get

    {

        return EventMsg;

    }

    set

    {

        EventMsg = value;

    }

}



// Constructor for setting the event message

public StatusChangedEventArgs(string strEventMsg)

{

    EventMsg = strEventMsg;

}

      }

      // This delegate is needed to specify the parameters we're passing with our event
      public delegate void StatusChangedEventHandler(object sender, StatusChangedEventArgs e);

      public static event StatusChangedEventHandler StatusChanged; Okay, so in the above code, we created a delegate and a custom event.

      // This is called when we want to raise the StatusChanged event

public static void OnStatusChanged(StatusChangedEventArgs e)

{

    **StatusChangedEventHandler statusHandler = StatusChanged;**

    if (statusHandler != null)

    {

        // Invoke the delegate

        statusHandler(null, e);

    }

}

      What does the bold line do? Why do we check for null? EDIT: The code is from C# Chat: Part 2 - Building the Chat Server (C# Programming Tutorial) • Geekpedia[^] check that out, if the above code doesn't provides what I mean.

      Top Web Hosting Providers[^] Do, or do not. There is no 'try'.

      D Offline
      D Offline
      DaveyM69
      wrote on last edited by
      #3

      Luc's answer is perfect. In addition, there's no need to declare your own delegate (from 2.0 onwards). Instead, just declare the event... public event EventHandler<StatusChangedEventArgs> StatusChanged; and in the OnStatusChanged... EventHandler<StatusChangedEventArgs> statusHandler = StatusChanged;

      Dave
      BTW, in software, hope and pray is not a viable strategy. (Luc Pattyn)
      Visual Basic is not used by normal people so we're not covering it here. (Uncyclopedia)

      L 1 Reply Last reply
      0
      • L Luc Pattyn

        Hi, you make a local copy of StatusChanged, so the same value gets used both in the null-text and the invocation; StatusChanged is public, so it could be changed by another thread at any point in time; removing the last handler from it right after the null-test got executed would cause an exception in statusHandler(null, e); In summary, without it your code would not be thread-safe. :)

        Luc Pattyn [Forum Guidelines] [My Articles]

        - before you ask a question here, search CodeProject, then Google - the quality and detail of your question reflects on the effectiveness of the help you are likely to get - use the code block button (PRE tags) to preserve formatting when showing multi-line code snippets

        modified on Friday, June 10, 2011 12:01 PM

        S Offline
        S Offline
        staticv
        wrote on last edited by
        #4

        so the same value gets used both in the null-text and the invocation; I'm sorry, but I didn't got it, what do you mean by null-text and the invocation? I'm converting this code to C++/CLI so I don't know C#, that's why having little problems... :)

        Top Web Hosting Providers[^] Do, or do not. There is no 'try'.

        L 1 Reply Last reply
        0
        • S staticv

          // Holds the arguments for the StatusChanged event

          public class StatusChangedEventArgs : EventArgs

          {

          // The argument we're interested in is a message describing the event

private string EventMsg;



// Property for retrieving and setting the event message

public string EventMessage

{

    get

    {

        return EventMsg;

    }

    set

    {

        EventMsg = value;

    }

}



// Constructor for setting the event message

public StatusChangedEventArgs(string strEventMsg)

{

    EventMsg = strEventMsg;

}

          }

          // This delegate is needed to specify the parameters we're passing with our event
          public delegate void StatusChangedEventHandler(object sender, StatusChangedEventArgs e);

          public static event StatusChangedEventHandler StatusChanged; Okay, so in the above code, we created a delegate and a custom event.

          // This is called when we want to raise the StatusChanged event

public static void OnStatusChanged(StatusChangedEventArgs e)

{

    **StatusChangedEventHandler statusHandler = StatusChanged;**

    if (statusHandler != null)

    {

        // Invoke the delegate

        statusHandler(null, e);

    }

}

          What does the bold line do? Why do we check for null? EDIT: The code is from C# Chat: Part 2 - Building the Chat Server (C# Programming Tutorial) • Geekpedia[^] check that out, if the above code doesn't provides what I mean.

          Top Web Hosting Providers[^] Do, or do not. There is no 'try'.

          G Offline
          G Offline
          Giorgi Dalakishvili
          wrote on last edited by
          #5

          Here is a very good article about events: C# Event Implementation Fundamentals, Best Practices and Conventions[^]

          Giorgi Dalakishvili #region signature My Articles Asynchronous Registry Notification Using Strongly-typed WMI Classes in .NET [^] My blog #endregion

          1 Reply Last reply
          0
          • L Luc Pattyn

            Hi, you make a local copy of StatusChanged, so the same value gets used both in the null-text and the invocation; StatusChanged is public, so it could be changed by another thread at any point in time; removing the last handler from it right after the null-test got executed would cause an exception in statusHandler(null, e); In summary, without it your code would not be thread-safe. :)

            Luc Pattyn [Forum Guidelines] [My Articles]

            - before you ask a question here, search CodeProject, then Google - the quality and detail of your question reflects on the effectiveness of the help you are likely to get - use the code block button (PRE tags) to preserve formatting when showing multi-line code snippets

            modified on Friday, June 10, 2011 12:01 PM

            J Offline
            J Offline
            jkohler
            wrote on last edited by
            #6

            Isn't that just making a copy of the reference? And therefore no safer than testing/invoking directly on the original reference?

            Subvert The Dominant Paradigm -- bumper sticker, circa 1971

            L 1 Reply Last reply
            0
            • S staticv

              so the same value gets used both in the null-text and the invocation; I'm sorry, but I didn't got it, what do you mean by null-text and the invocation? I'm converting this code to C++/CLI so I don't know C#, that's why having little problems... :)

              Top Web Hosting Providers[^] Do, or do not. There is no 'try'.

              L Offline
              L Offline
              Luc Pattyn
              wrote on last edited by
              #7

              :confused: :confused:

              if (statusHandler != null) <--- that looks like a null-test
              // Invoke the delegate
              statusHandler(null, e); <--- according to the comment this is an invocation

              :)

              Luc Pattyn [Forum Guidelines] [My Articles]

              - before you ask a question here, search CodeProject, then Google - the quality and detail of your question reflects on the effectiveness of the help you are likely to get - use the code block button (PRE tags) to preserve formatting when showing multi-line code snippets

              modified on Friday, June 10, 2011 12:02 PM

              S 1 Reply Last reply
              0
              • D DaveyM69

                Luc's answer is perfect. In addition, there's no need to declare your own delegate (from 2.0 onwards). Instead, just declare the event... public event EventHandler<StatusChangedEventArgs> StatusChanged; and in the OnStatusChanged... EventHandler<StatusChangedEventArgs> statusHandler = StatusChanged;

                Dave
                BTW, in software, hope and pray is not a viable strategy. (Luc Pattyn)
                Visual Basic is not used by normal people so we're not covering it here. (Uncyclopedia)

                L Offline
                L Offline
                Luc Pattyn
                wrote on last edited by
                #8

                :rose:

                Luc Pattyn [Forum Guidelines] [My Articles]

                - before you ask a question here, search CodeProject, then Google - the quality and detail of your question reflects on the effectiveness of the help you are likely to get - use the code block button (PRE tags) to preserve formatting when showing multi-line code snippets

                1 Reply Last reply
                0
                • L Luc Pattyn

                  :confused: :confused:

                  if (statusHandler != null) <--- that looks like a null-test
                  // Invoke the delegate
                  statusHandler(null, e); <--- according to the comment this is an invocation

                  :)

                  Luc Pattyn [Forum Guidelines] [My Articles]

                  - before you ask a question here, search CodeProject, then Google - the quality and detail of your question reflects on the effectiveness of the help you are likely to get - use the code block button (PRE tags) to preserve formatting when showing multi-line code snippets

                  modified on Friday, June 10, 2011 12:02 PM

                  S Offline
                  S Offline
                  staticv
                  wrote on last edited by
                  #9

                  Hmmm, you wrote null-text :) lol.. Thanks anyways

                  Top Web Hosting Providers[^] Do, or do not. There is no 'try'.

                  1 Reply Last reply
                  0
                  • J jkohler

                    Isn't that just making a copy of the reference? And therefore no safer than testing/invoking directly on the original reference?

                    Subvert The Dominant Paradigm -- bumper sticker, circa 1971

                    L Offline
                    L Offline
                    Luc Pattyn
                    wrote on last edited by
                    #10

                    Yes, it copies a reference, making sure the event and the delegates inside it can not be garbage collected (it is a local copy). There still is a possibility that the event gets changed (it is kind of a linked list), to cope with that you could (probably should) add a try-catch around the invocation. This is one of those rare cases where an empty catch block would be fine: after all, if another thread is allowed to remove the last delegate, that really means there is no need to call it anymore. Have a look at the fine article Giorgi provided. :)

                    Luc Pattyn [Forum Guidelines] [My Articles]

                    - before you ask a question here, search CodeProject, then Google - the quality and detail of your question reflects on the effectiveness of the help you are likely to get - use the code block button (PRE tags) to preserve formatting when showing multi-line code snippets

                    modified on Friday, June 10, 2011 12:02 PM

                    J 1 Reply Last reply
                    0
                    • L Luc Pattyn

                      Yes, it copies a reference, making sure the event and the delegates inside it can not be garbage collected (it is a local copy). There still is a possibility that the event gets changed (it is kind of a linked list), to cope with that you could (probably should) add a try-catch around the invocation. This is one of those rare cases where an empty catch block would be fine: after all, if another thread is allowed to remove the last delegate, that really means there is no need to call it anymore. Have a look at the fine article Giorgi provided. :)

                      Luc Pattyn [Forum Guidelines] [My Articles]

                      - before you ask a question here, search CodeProject, then Google - the quality and detail of your question reflects on the effectiveness of the help you are likely to get - use the code block button (PRE tags) to preserve formatting when showing multi-line code snippets

                      modified on Friday, June 10, 2011 12:02 PM

                      J Offline
                      J Offline
                      jkohler
                      wrote on last edited by
                      #11

                      ahhh... reference to empty versus null reference. Got it. Now I have to rework a whole bunch of code... I foresee a generic solution

                      Subvert The Dominant Paradigm -- bumper sticker, circa 1971

                      1 Reply Last reply
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