Hi,

I'm not sure how to reproduce your exact problem.  But DataBinding does
attach an eventhandler to the Validating event inwhere it will push control
values to datasource values but only if they are changed (if the property
has a similar changed event).

1) If you are overriding OnValidating, then you should call
Base.OnValiditing at the end since this will trigger the Validating event
and a data push from control to source.  Something like this seems to work:

public class Class1 : TextBox
{
 private Decimal val;

 public event EventHandler ValueChanged;

 public Decimal Value
 {
   get { return val; }
   set
   {
      if (val != value)
      {
         val = value;
         Text = val.ToString();
         if (ValueChanged != null)
           ValueChanged(this, EventArgs.Empty);
      }
    }
 }

 protected override void OnValidating(System.ComponentModel.CancelEventArgs
e)
 {
    decimal result;
    if (decimal.TryParse(Text, out result))
    {
       Value = result;
       BackColor = System.Drawing.Color.White;
    }
    else
   {
       BackColor = System.Drawing.Color.Red;
       e.Cancel = true;
   }
   base.OnValidating(e);
 }
}

2) But if you would bind to a normal TextBox Text property,  then
DataBinding will try to parse the Text into a Decimal also and if it fails
you can do something inside Binding.BindingComplete eventhandler:

textBox1.DataBindings["Text"].BindingComplete += new
BindingCompleteEventHandler(Form3_BindingComplete);

void Form3_BindingComplete(object sender, BindingCompleteEventArgs e)
{
 if (e.BindingCompleteContext == BindingCompleteContext.DataSourceUpdate)
 {
  if (e.BindingCompleteState != BindingCompleteState.Success)
     e.Binding.Control.BackColor = System.Drawing.Color.Red;
  else
     e.Binding.Control.BackColor = System.Drawing.Color.White;
 }
}

For both cases to work properly you need to make sure formatting is enabled
(code or desinger) (Binding.FormattingEnabled).

HTH,
Greetings