As you should know already, every class has a fundamental method called a default constructor. Every control of the .NET Framework is based on a class that has a default constructor and most of those classes have only one constructor: the default. The default constructor allows you to instantiate the class without necessarily initializing it. To use it, you must know the name of the control you want to use since each control bears the same name as its class. Here is an example:
Imports System Imports System.Drawing Imports System.Windows.Forms Module Exercise Public Class WinControls Inherits Form Private BtnReset As Button Dim components As System.ComponentModel.Container Public Sub New() InitializeComponent() End Sub Public Sub InitializeComponent() BtnReset = New Button() End Sub Public Shared Function Main() As Integer Application.Run(New WinControls()) Return 0 End Function End Class End Module
If you are not planning to use a control straight from the .NET Framework, you can also create your own class that is derived from the class of the control, as we have mentioned in previous lessons.
As mentioned in the previous lesson, after instantiating a control, it is available but the user cannot see. Each control that acts as a parent of another control has a property called Controls. This property, which is a ControlCollection type, is equipped with an Add() method. If you want to display the new control to the user, you should pass it to the Control.Controls.Add() method. Here is an example:
Imports System Imports System.Drawing Imports System.Windows.Forms Module Exercise Public Class WinControls Inherits Form Private BtnReset As Button Dim components As System.ComponentModel.Container Public Sub New() InitializeComponent() End Sub Public Sub InitializeComponent() BtnReset = New Button() Controls.Add(BtnReset) End Sub Public Shared Function Main() As Integer Application.Run(New WinControls()) Return 0 End Function End Class End Module
This displays the control to the user.
After using a control, it must be destroyed. Another detail of Windows controls is that they use or consume computer resources during their lifetime. When the controls are not used anymore, such as when their application closes, these resources should be freed and given back to the operating system to make them available to other controls. This task can be performed using the Dispose() method to the Control class, which can then be overridden by its child controls. The syntax of the Control.Dispose() method is:
Protected Overrides Sub Dispose(disposing As Boolean)
This method takes one argument, disposing, that indicates how the resources would be released. If this argument is passed with a False value, only the unmanaged resources would be released. If it is passed as True, then both managed and unmanaged resources would be released.
In the previous lesson, we saw that you could call the Visible property to hide a visible control or to display a hidden control. Besides the Visible property, you can also display a control using the Show() method. Its syntax is:
Public Sub Show()
When you use this method, if the control is visible, nothing would happen. If it were hidden, then it would be revealed. The Show() method internally sets the Visible property to true. We also saw that, to hide a control, you could set its Visible property to False. In the same way, to hide a control, you call can the Control.Hide() method. Its syntax is:
Public Sub Hide()
Keep in mind that hiding a control does not destroy it.
Once a control is visible, the user can use it. Some controls allow the user only to read their text or view what they display. Some other controls allow the user to retrieve their value or to change it. To perform such operations, the user must first give focus to the control. The focus is a visual aspect that indicates that a control is ready to receive input from the user. Various controls have different ways of expressing that they have received focus.
Button-based controls indicate that they have focus by drawing a dotted rectangle around their caption. In the following picture, the button on the right has focus:
A text-based control indicates that it has focus by displaying a blinking caret. A list-based control indicates that it has focus when one of its items has a surrounding dotted rectangle:
To give focus to a control, the user can press a key such as Tab. To programmatically give focus to a control, call the Focus() method.
In the previous lesson, we saw how you could position controls visually or manually. In some cases, you end up with one control positioned on top of another. Of course the first remedy that comes in mind would consist of moving one of the controls away from the other. This would also imply sometimes that you would have to enlarge and/or heighten the controls' container. There are situations that either you don't want to resize the parent or you can't: your only solution is to have one control on top of another. Fortunately, you can cope with this situation by specifying, when needed, what control at what time should be displayed to the user.
When one control is positioned on top of another, they use a third axis whose origin, like that or the other axes, is on the top-left corner of the parent. This third axis, also considered the z-axis, is oriented so that it moves from the monitor towards the user. The operating system is in charge of positioning and drawing the objects on the screen. You as the programmer can direct the operating system as to what object should be on top and what object should be behind. To support these changes the Control class uses two methods that its appropriate children derive also.
When a control A is positioned behind a control be, this causes control be either partially or completely hidden. If you want the control A to change its z-order and become on top of control B, you can call its BringToFront() method. The syntax of this method is:
Public Sub BringToFront()
On the other hand, if a control B is positioned on top of a control A, if you want control B to become positioned behind control A, you can call control B's SendToBack() method. Its syntax is:
Public Sub SendToBack()
After the controls have been positioned, at any time, when you access a control, if you want to know whether that control is the most top object, you can call its GetTopLevel() method. Its syntax is:
Protected Function GetTopLevel As Boolean
The Windows controls available from the .NET Framework and that we will user in our lessons. They are equipped with various methods ready to be used. Of course, no library can surely provide every single type of method that every programmer would use. For this reason, it will not be unusual that you need a method that is not available for a control you are using. In the same way, when you create a Windows Application that is based on a Form class, you will likely need a method that is not defined in the Form class. In this case, you can create your own and new method.
A method is created like a normal procedure. If you want to add it to a form, you can open the Code Editor and write your procedure outside of any existing procedure. Here is an example:
Imports System Imports System.Drawing Imports System.Windows.Forms Module Exercise Public Class WinControls Inherits Form Private BtnReset As Button Dim components As System.ComponentModel.Container Public Sub New() InitializeComponent() End Sub Public Sub InitializeComponent() BtnReset = New Button() BtnReset.Text = "Reset" BtnReset.Location = New Point(20, 20) Controls.Add(BtnReset) End Sub Private Function CalculateRectangleArea(ByVal Recto As Rectangle) As Double Return Recto.Width * Recto.Height End Function Public Shared Function Main() As Integer Application.Run(New WinControls()) Return 0 End Function End Class End Module
In the same way, if you derive a class from one of the existing classes because you want to get a custom control from it, you can declare a new method as you see fit and use it appropriately.
Probably the best way is to let the Code Editor insert the new method based on your specifications. To do that, in the Class View, first expand the name of the project. Then, right-click the name of the class where you want to add a new method, position the mouse on Add, and click Add Method. This would open the C# Method Wizard dialog box you can fill out and click Finish. After the method's body has been defined you
Among all the languages of the .NET Framework and Microsoft Visual Studio, Visual Basic has the largest and the most impressive library of functions. There are so many of these functions, we cannot review them here.