Is it possible to subscribe one control to the same event handler multiple times?

I would like to know if I bind the TextChanged event handler to a TextBox control, then how can I ensure that w开发者_运维百科on't be allowed to bind this event handler again?

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You can't ensure that. You would theoretically be allowed to bind the same event handler to a textbox (or other control) more than once. The only thing that events allow you to do is add a handler and remove a handler—there's no additional means provided to check for existing subscribers. If you don't believe me, Jon Skeet provides the authoritative answer here, and in his article on events.

If you need to ensure that you don't accidentally subscribe a control to the same event twice, you'll need to keep track of it yourself. Honestly, you should never end up in a situation where you don't know what event handlers are subscribed. Not only does this reflect sloppy design, but it probably also means that you aren't taking care to remove your event handlers when they are no longer necessary.

A possible solution is provided in the answers to this question, but I caution you from using something like this blindly. As others have argued, this code is something of an anti-pattern.

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You can bind it programatically as many times as you'd like. If you want to prevent this you can use a List<object> to keep references in, for example:

   
   private List gotRefs = new List();
   public void MyMethod()
   {
      if (!gotRefs.Contains(txtTextBox1)) {
        txtTextBox1.TextChanged += txtTextBox1_TextChanged;
        gotRefs.Add(txtTextBox1);
      }
   }

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You can actually use a hack.

• You can make textbox private to your class to ensure, that only your single class is able to access the text box and add handlers to it.

private TextBox txtChanging;

• Then, in your class, you can create a custom event like textBoxTextChanged

public event Action TextBoxTextChanged;

• Create a standart method OnTextBoxTextChanged

private OnTextBoxChanged( object sender, EventArgs args )
{
    if( TextBoxTextChanged != null )
        TextBoxTextChanged();
}

• Bind this OnTextBoxChangedMethod to TextChanged event of the TextBox

txtChanging.TextChanged += OnTextBoxChanged;

• Ensure that NO OTHER METHOD is bound to TextChanged event of the text box

• Bound all your main payload text changed handlers to your new custom event instead of text box textchanged event directly

TextBoxTextChanged += txtChanged_TextChnaged;

• All that paperwork is because events actually provide a lot of information, but only to the methods inside the class where they are defined.

• You can check bound delegates with

TextBoxTextChanged.GetInvocationList()

and their count with

TextBoxTextChanged.GetInvocationList().Count() //(C# 4.0/LINQ)

or just count them through foreach.