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Property Notification

Posted on 2004-09-19
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Last Modified: 2010-04-15
Hello, Is there a way I can create a property in a class (eg. public int m_nNumber), and somehow create an event handler which is called each time this property is changed?
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Question by:auk_ie
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7 Comments
 
LVL 10

Assisted Solution

by:eternal_21
eternal_21 earned 200 total points
ID: 12099011

  public event System.EventHandler NumberChanged;

  private int number;
  public int Number
  {
    get
    {
      return number;
    }
    set
    {
      if(number!=value)
      {
        number = value;
        if(NumberChanged!=null)
          NumberChanged(null, null);
      }
    }
  }
0
 
LVL 5

Accepted Solution

by:
tzxie2000 earned 800 total points
ID: 12099406
//define event args

  public class NumChangedEventArgs : EventArgs
  {
    private readonly int num;
    public int m_num
    {
      get{return num;}
    }
    public NumChangedEventArgs(int num)
    {
      this.num = num;
    }
  }

//define delegate
  public delegate void NumChangedEventHandler(object sender, NumChangedEventArgs e);

//define your class
  public class testclass
  {
    public testclass()
    {
      nNumber=0;
    }
    private int nNumber;
    public int m_nNumber
    {
      get{return nNumber;}
      set{nNumber=value;OnNumChanged(new NumChangedEventArgs(value));}//call the event
    }
    public event NumChangedEventHandler NumChanged;//event handle
    protected virtual void OnNumChanged(NumChangedEventArgs e)//event entry
    {
      if (NumChanged != null)
      {
        // Invokes the delegates. NumChangedEventArgs
        NumChanged(this, e);
      }
    }
  }

when using

you can write this in a button click event


private void button1_Click(object sender, System.EventArgs e)
{
   testclass test=new testclass();//new class
   test.NumChanged+=new NumChangedEventHandler(NumChanged);// assigned event
   test.m_nNumber=10;//change number
}

public void NumChanged(object sender, NumChangedEventArgs e)//event action
{
  MessageBox.Show(e.m_num.ToString());//show message
}
0
 

Author Comment

by:auk_ie
ID: 12099820
Nice answer tzxie2000 (a little elaborate for my needs but very good)

eternal_21, How would you write a handler for the public event System.EventHandler NumberChanged;

I tried

public void NumberChanged(object sender, NumChangedEventArgs e)//event action
{
  MessageBox.Show(number.ToString());
}

But that doen't compile

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LVL 10

Expert Comment

by:smegghead
ID: 12102737
just use System.EventArgs rather than NumChangedEventArgs

public void NumberChanged(object sender, System.EventArgs e)//event action
{
  MessageBox.Show(number.ToString());
}
0
 

Author Comment

by:auk_ie
ID: 12105493
Yes this is what I had,


public event System.EventHandler NumberChanged; //What does this line do.

//Duplicate declaration
public void NumberChanged(object sender, System.EventArgs e)//event action
{
  MessageBox.Show(number.ToString());
}
0
 
LVL 10

Expert Comment

by:smegghead
ID: 12105518
so, what compile error did you get ?
0
 
LVL 5

Expert Comment

by:tzxie2000
ID: 12105716
I think you'd better use the full version of event
or you may simplely write a function to call it

//define a function
void NumberChanged(int number)
{
 //doing something
}//if in a class you can overwrite it for different usage

 public int Number
  {
    get
    {
      return number;
    }
    set
    {
        number = value;
        NumberChanged(number);
    }
  }
0

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