Help needed: Custom data-bound control modifies Rows just by navigating through DataTable

26 Oct 2006 11:41 PM

herceg

I have a problem with a custom data-bound control which inherits from
TextBox. Its code is basically

class MyBox: TextBox
{
    public string MyText
    {
        get { return base.Text; }
        set { base.Text = value; }
    }
}

and the only difference between MyBox and TextBox is that MyBox has a
blue background :)

I put MyBox on a form which also holds a DataSet and a BindingSource.
Then I bind MyBox.MyText to a field in a DataTable. When I use it on
one record, it works fine (MyBox shows value in the field and by typing
in it I can modify the value).

But when I try navigating through the DataTable using the
BindingSource.MovePrevious and .MoveNext methods, DataRows get modified
even if I don't change anything (by typing in MyBox). I then replace
MyBox with TextBox, bind it the same way and navigation doesn't modify
rows, which is expected behavior.

What is different between the original TextBox and MyBox? Why does
MyBox modify rows?

I detect modified rows by calling GetChanges and checking the
Rows.Count property, and every time I navigate to another row, it gets
modified.

I suppose I should implement something to support data change
notifications in MyBox class, but I don't know what.

Please help!

27 Oct 2006 7:55 AM

herceg

OK, I found the solution to this in MSDN. The article is called

    Windows Forms Programming
    How to: Add Change Notification for Data Binding

Unfortunately, the code in MSDN article doesn't work because they
forgot (?) an "event" keyword which is crucial in this case. My MSDN
Library is from 2005, but on the online MSDN library
(http://msdn2.microsoft.com/en-us/library/ms229615.aspx) they have the
same old sample without the "event" keyword. Do they check their
samples at all? I lost two days trying to figure this out.

The correct code follows:

    class MyBox: TextBox
    {
        public MyBox(): base()
        {
            // when users type something in the base TextBox, MyBox has
to update itself
            base.TextChanged += new EventHandler(MyBox_TextChanged);
        }

        void MyBox_TextChanged(object sender, EventArgs e)
        {
            this.MyText = base.Text;
        }

        // This event MUST BE declared. Note the "event" keyword!
        public event EventHandler MyTextChanged;

        private void OnMyTextChanged()
        {
            if (MyTextChanged != null)
            {
                MyTextChanged(this, new EventArgs());
            }
        }

        // And this is the bindable property "MyText"
        [Bindable(BindableSupport.Yes, BindingDirection.TwoWay)]
        public string MyText
        {
            get
            {
                return base.Text;
            }
            set
            {
                base.Text = value;
                OnMyTextChanged();
            }
        }

    }