Visual Inheritance in VB.NET

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We ve seen how useful inheritance can be in the creation of new classes that can inherit the data and behaviour of existing classes Visual inheritance is analogous to this in form design To use it, you rst create a Class Library that includes a form design and compile this (Visual Studio will create a dll (Dynamic Link Library) le By then adding this library to a new project, the form design included in the class library will be available as an inherited form in the new application Any controls of code you added to the form design in the Class Library will be present on the inherited form For example, you might decide that every dialog box you create in a new application should have an OK and a Cancel button like the ones used in dlgPerson in Figure 89; this is a sensible decision since users will have a consistent way of dismissing every dialog box that they meet in the application Instead of having to add the buttons to every dialog box, set their and the form s properties to con gure them and associate them with the form s dialog actions, and then add click eventhandlers to trigger the closing of the form, we can create a new form design that is already set up in this way, and inherit from it
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A standard form of Dialog Box
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The easiest starting point for a Visual Form design that you can inherit from is to start a new WinForms project and con gure the default form in this to be the form you will inherit from The form design shown in Figure 810 is that for a simple dialog box with OK and Cancel buttons The required property settings are shown in Table 84 Figure 810 shows a standard design for a dialog box Buttons have been given a suitable size and position to be OK and Cancel buttons (Windows users expect these buttons at the bottom of a dialog box, with OK to the left and Cancel to the right),
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Table 84 Property settings for the frmDialog inherited form
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Control Form
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Property Name ContextMenu AcceptButton CancelButton
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Setting frmDialog
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btnOK (Note must add and rename the
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button before making this setting)
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btnCancel (Note must add and rename
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the button before making this setting)
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Name Caption Anchor DialogResult Name Caption Anchor DialogResult
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btnOK &OK Bottom, Left OK btnCancel Cancel Bottom, Right Cancel
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Button
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and have had their DialogResult properties set appropriately The form has been associated with them by setting its AcceptButton and CancelButton properties to refer to them Luckily, controls also have an Anchor property that is used to make them stay a xed distance away from two edges of the form they are on: this property, set to Bottom, Left for the OK button, and Bottom, Right for the Cancel button will make sure the buttons stay in the proper position relative to the form even when it is resized Finally, code can be added to the form class to make the buttons able to dismiss the form from the screen, as shown in Listing 819
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Public Class frmDialog Inherits SystemWindowsFormsForm + Windows Form Designer generated code Private Sub btnOK_Click(ByVal sender As SystemObject, _ ByVal e As SystemEventArgs) _ Handles btnOKClick MeHide() End Sub Private Sub btnCancel_Click(ByVal sender As SystemObject, _ ByVal e As SystemEventArgs) _ Handles btnCancelClick MeHide() End Sub End Class Listing 819: The code needed for the frmDialog class
Once the form design is complete to your satisfaction, you can test the project to ensure all is ok When it runs, you should be able to dismiss the form by pressing either of the buttons Now, to convert this into a form that you can inherit, go to the Project Properties dialog (select the project in the solution explorer and then Project Properties from the menus) and in the Common Properties, General page, change the Output Type to Class Library as shown in Figure 811 Finally, Build the Assembly (Build Build Solution from the menus) and close the solution down You can now test the inherited form by creating a new project and Inheriting a form Create a new WinForms project To add the inherited form, right click on the pro ject in the Solution Explorer, and select Add Inherited Form from the context menu In the dialog box that appears, make sure Local Project Items is selected in the left hand pane, and Inherited Form is selected on the right hand one Give the new form a name (eg dlgTest) and click Open The dialog box that appears now may look initially as if you are being warned of an error, but click on its Browse button and then navigate your way to the folder that contains the project with the dialog box design choose the DLL le from the BIN folder to reference the Class Library, and select the form design from that project A new form matching the design you saved to inherit from will have been added to the project Controls on the form will be marked with a tiny inherited forms icon