TextChanged Events - Why does this not result in an infinite loop?

Question

While trying to do something a bit more complicated, I ran across a behavior I don't quite understand.

Assume the following code below handling the textChanged event.

 private void textChanged(object sender, TextChangedEventArgs e)
    {
        TextBox current = sender as TextBox;
        current.Text = current.Text + "+";
    }

Now, typing a character in the textbox (say, A) will result in the event getting tripped twice (adding two '+'s) with the final text displayed being just A+.

My two questions are, why is the event hit just twice? And why does only the first run through the event actually set the text of the textbox?

Thanks in advance!

Answer 1

Well - setting the Text property while it is being changed / while it has just changed seems to be caught by the TextBox class explicitly:

Just use the Reflector to look inside TextBox.OnTextPropertyChanged (shortened):

TextBox box = (TextBox) d;
if (!box._isInsideTextContentChange)
{
    string newValue = (string) e.NewValue;
    //...
    box._isInsideTextContentChange = true;
    try
    {
        using (box.TextSelectionInternal.DeclareChangeBlock())
        {
           //...
        } //Probably raises TextChanged here
    }
    finally
    {
        box._isInsideTextContentChange = false;
    }
    //...
}

The field _isInsideTextContentChange is set to true before the TextChanged event gets raised. When changing the Text property again, the TextChanged event thus is not raised again.

Therefore: Feature ;-)