Binding the UI to the Business Objects in .NET

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selectedID = DirectCast( _ CustomerListComboBoxSelectedValue, Integer) MePurchaseListBindingSourceDataSource = _ PurchaseListCreate(selectedID) End If End Sub
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This code retrieves the selected ID from the combo box and uses it to create the appropriate PurchaseList object It then assigns the DataSource property to this PurchaseList object instance Delete the code from the form s Load event that sets the BindingSource, because now it is set in the SelectedValueChanged event instead At this point, you have all the user interface and code in place for the CustomerWin, ProductWin, and PurchasesWin forms You also have the business object classes for Customer, CustomerList, Product, ProductList, Purchases, PurchaseList, and TypeList Run the application It displays your splash screen and then shows the MDI parent form You can select to view customers, products, and customer purchases Wow!
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If the user always did everything right and never did anything wrong, your user interface code would be complete But presuming perfection from the user is not a viable expectation for your application Instead, you need to follow defensive coding techniques and assume that the user won t put in valid data The best way to implement user-entry validation is to use the built-in object binding features Just add code to the properties in the business object classes to validate the property values Then use the ErrorProvider control error icon to display any errors to the user Using object binding, the NET runtime automatically sets the ErrorProvider error icon text based on your validation code
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The ErrorProvider is a build-in component that you add to your forms to display validation errors to the user If the user enters invalid data into a control on the form, the ErrorProvider component places an error icon next to the control When the user positions the cursor over the error icon, a ToolTip appears, displaying the error message for the control, as shown in Figure 716
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7 BINDING THE USER INTERFACE TO THE BUSINESS OBJECTS
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When a property is invalid, the ErrorProvider displays an error icon with ToolTip text describing the error
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Using the ErrorProvider is better than displaying an error in a message box, because once the message box is dismissed, the error message is no longer visible Plus, a message box interferes with the user s data entry ow Using the ErrorProvider, the user can enter all the data for the form and then look at which controls have an error icon without having the interruption of a message box You can implement the ErrorProvider functionality manually using methods of the ErrorProvider component to position the error icon and identify the error text But if you are using object binding, the ErrorProvider can be set up to work automatically To display an error message when using object binding:
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1 Open the form in the Forms Designer 2 Add the ErrorProvider control to the form The control appears in the component tray 3 In the Properties window for the ErrorProvider, set the DataSource property to the BindingSource for the business object associated with the form You may also want to turn off the icon blinking, because many users nd it irritating In the Properties window, set the BlinkStyle property of the ErrorProvider to NeverBlink Implementing an ErrorProvider control provides a very clean and user-friendly means of displaying data entry errors to the user And it works well with object binding
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For the Purchase Tracker sample application: Add the ErrorProvider control to each of the data entry forms: ProductWin and CustomerWin Bind each ErrorProvider to the business object BindingSource for the form For the ProductWin form, bind the ErrorProvider to the ProductBindingSource For the CustomerWin form, bind the ErrorProvider to the CustomerBindingSource The application does not yet provide any validation code, so you won t see an error icon appear yet when you run the application
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