Handling Validation Events in .NET

Draw Code 128A in .NET Handling Validation Events
Handling Validation Events
Painting Code 128C In VS .NET
Using Barcode creation for Visual Studio .NET Control to generate, create Code-128 image in VS .NET applications.
Every Windows Forms control exposes two events, Validating and Validated, which they inherit from the baseControl class When or whether these events ever fire depends on the design of the derived control, certain properties in the container control to which a control belongs, and what programmatic code is invoked with respect to validation The Validating event is intended to fire immediately after input has been completed, but before it has been accepted as valid The Validated event fires after the input has been accepted as valid The Validating event is the one you will handle most often for data-binding scenarios When a control decides that input is complete, typically because the focus is shifting to another control on the form, it should fire the Validating event This event is of type CancelEventHandler, which takes an event argument of typeCancelEventArgs The CancelEventArgs class contains a single Boolean property named Cancel that you can set to signal that the event being fired shouldn't be completed Setting Cancel to true is a signal back to the control that validation failed in the code that handles the event For example, say you want to write some code in a login form that checks a username field A simple example that you could write to confirm that some value was entered would be to subscribe to the Validating event for the TextBox control that takes the username with the following handler: private void OnUsernameValidating(object sender, CancelEventArgs e) { if (stringIsNullOrEmpty(m_UsernameTextBoxText)) { eCancel = true; MessageBoxShow("Username is a required field"); } }
Decode Code-128 In Visual Studio .NET
Using Barcode scanner for .NET framework Control to read, scan read, scan image in .NET applications.
This code is part of the form that contains the m_UsernameTextBox control and gets invoked by the form when the focus switches from that control to some other control on the form The code uses the IsNullOrEmpty method on the String class to check whether the text box is empty If so, it sets the CancelEventArgs argument's Cancel property to true Setting this argument to true is a signal to the validation infrastructure of Windows Forms that validation has failed on that control, which will terminate the validation process by default The default value of Cancel is false, which allows the Validation process to continue A message box is then shown to give the user some (crude) feedback about what the problem is By default, a couple of things happen when you set the CancelEventArgsCancel property to true in your Validating event handler The first is that the focus won't leave that control, forcing the user to correct the problem before being able to move on to input data in other controls (see Figure 101) This may be a good thing in many situations, because it makes it so users can't get too far out of context from where they made an input error before correcting it This will also prevent the Validated event from firing since the validation process didn't complete
Create Barcode In Visual Studio .NET
Using Barcode printer for .NET Control to generate, create barcode image in VS .NET applications.
Figure 101 Control Validation Process
Reading Bar Code In VS .NET
Using Barcode scanner for .NET framework Control to read, scan read, scan image in .NET framework applications.
This document was created by an unregistered ChmMagic, please go to http://wwwbisentercom to register it Thanks
Make Code 128B In C#.NET
Using Barcode creator for .NET framework Control to generate, create Code 128A image in Visual Studio .NET applications.
However, there are several problems with this approach First, you may not always want to force users to correct their errors immediately; you may want to let them complete an entire form of entries and just force them to resolve any problems before submitting or saving the data This allows rapid data entry for people who spend their days repeatedly filling out the same form over and over In those cases, if they are tabbing from field to field, they don't have to constantly look at the form to see if the focus wasn't allowed to shift to the next control because of a validation failure Another problem with this approach is that if users try to close a form that has validation errors that are being handled by canceling the Validating event, by default they won't be able to close the form The act of clicking on another control, such as the window frame buttons (the X button), causes a focus change, which triggers validation, which fails and sets the focus back onto the control that failed Finally, this approach requires that the control first obtain the focus, then give up the focus to another control before the validation process will be invoked Luckily NET 20 introduces the AutoValidate property on the Form class that lets you specify exactly what the behavior should be when a validation error occurs at the control level This property is discussed later in this chapter
Print Code 128C In .NET
Using Barcode creation for ASP.NET Control to generate, create Code-128 image in ASP.NET applications.
This document was created by an unregistered ChmMagic, please go to http://wwwbisentercom to register it Thanks
Code 128A Drawer In VB.NET
Using Barcode drawer for .NET framework Control to generate, create Code 128 Code Set C image in .NET applications.
Creating Barcode In VS .NET
Using Barcode drawer for .NET Control to generate, create bar code image in Visual Studio .NET applications.
Print EAN / UCC - 13 In VS .NET
Using Barcode generation for Visual Studio .NET Control to generate, create EAN 128 image in .NET framework applications.
EAN8 Printer In .NET Framework
Using Barcode creator for .NET Control to generate, create EAN 8 image in VS .NET applications.
Reading UPC Symbol In Visual Studio .NET
Using Barcode decoder for .NET Control to read, scan read, scan image in Visual Studio .NET applications.
Generating ANSI/AIM Code 39 In Java
Using Barcode generation for Java Control to generate, create Code39 image in Java applications.
Universal Product Code Version A Creation In Java
Using Barcode generator for Java Control to generate, create UPC-A Supplement 2 image in Java applications.
Bar Code Generator In Java
Using Barcode creator for Java Control to generate, create bar code image in Java applications.