Tech Tip: Use Validate() to Complete DataBinding

Level 200
DevForce Express

June 1, 2006

Here is a DataBinding "bug" that drives you nuts. Your "First Name" TextBox is bound to your Employee object's "FirstName" property. You change the TextBox value from "Nancy" to "Sally" and then click the Save button in the ToolStripMenu at the top of your form.

You later discover that your change was not saved; the first name is still "Nancy". You are sure you made that change and it said "Sally" on screen. So you change the name again, tab out of the FirstName TextBox, and click the Save button. You re-read the Employee and her first name is "Sally". What is going on?

The FirstName DataBinding did not complete when you clicked the ToolStrip's Save button. The TextBox value, "Sally", was not passed through to the Employee object's FirstName property so that property value still read "Nancy" when your application tried to save it.

Default DataBinding does not update the bound data item property until the control loses focus. The FirstName TextBox did not lose focus when you clicked the ToolStrip Save button. It only lost focus when you tabbed out of the TextBox, which is why your second attempt worked.

ToolStrip buttons are different in this respect from the buttons you drag onto the Form. When you click a Form button, the TextBox loses focus, the button gains focus, the DataBinding completes, and "Sally" makes it to the database.

ToolStrip buttons do not gain focus and the current control does not lose focus. Therefore, by default, DataBinding does not complete and the bound data item property remains unchanged.

Why are ToolStrip buttons different? Because you might want the button action to do something other than commit the TextBox value. The button might make the selected text bold, it might correct your spelling, or it might send out for room service. The .NET designers did not want to presume too much.

You must add a line of code to your button handler to ensure that a pending DataBinding completes. The handler must tell the Form (or UserControl) to "validate" itself by calling its Validate() method.

Sample shown below:

C#:

void SaveButtonHandler(object sender, EventArgs e) {
  // Forces current control to perform databinding as if it lost focus.
 this.Validate();
 this.PerformSave();
}

VB.NET:

Sub SaveButtonHandler(ByVal sender As object, ByVal e As EventArgs)
  'Forces current control to perform databinding as if it lost focus.
 Me.Validate()
 Me.PerformSave()
End Sub