October 17, 2009

Properties, Methods and Events

All the controls in the ToolBox except the Pointer are objects in Visual Basic. These objects have associated properties, methods and events.
Real world objects are loaded with properties. For example, a flower is loaded certain color, shape and fragrance. Similarly programming objects are loaded with properties. A property is a named attribute of a programming object. Properties define the characteristics of an object such as Size, Color etc. or sometimes the way in which it behaves. For example, a TextBox accepts properties such as Enabled, Font, MultiLine, Text, Visible, Width, etc.

  • Enables property allows the TextBox to be enabled or disabled at run time depending on the condition set to True or False.

  • Font property sets a particular font in the TextBox.

  • MultiLine property allows the TextBox to accept and display multiple lines at run time.

  • Text property of the TextBox control sets a particular text in the control.

  • Visible property is used to hide the object at run time.

  • Width property sets the TextBox to the desired width at design time.

The properties that are discussed above are design-time properties that can be set at the design tme by selecting the Properties Window. But certain properties cannot be set at desgn time. For example, the CurrentX and CurrentY properties of a Form cannot be set at the design time.
A method is an action that can be performed on objects. For example, a cat is an object. Its properties might include long white hair, blue eyes, 3 pounds weight etc. A complete definition of cat must only encompass on its looks, but should also include a complete itemization of its activities. Therefore, a cat's methods might be move, jump, play, breath etc.
Siimilarly in object-orinted programming, a method is a connected or built-in procedure, a block of code that can be invoked to impart some action on a particular object. A method requires an object to provide them with a context. For example, the word Move has no meaning in Visual Basic, but the statement,
Text1.Move 700, 400
performs a very precise action. The TextBox control has other associated methods such as Refresh, SetFocus, etc.

  • The Refresh method enforces a complete repaint of the control or a Form. For example, Text1.Refresh refreshes the TextBox.

  • The Setfocus method moves the focus on the control. For Example Text1.SetFocus sets the focus to TextBox control Text1.

Event Driven Programming

Visual Basic programs are built around events. Events are various things that can happen in a program. this will become clearer when studied in contrast to procedural programming. In procedural languages, an application is written is executed by checking for the program logically through the program statements, one after another. For a temporary phase, the control may be transferred to some other point in a program. While in an event driven application, the program statements are executed only when a particular event calls a specific part of the code that is assigned to the event.
Let us consider a TextBox control and a few of its associated events to understand the concept of event driven programming. The TextBox control supports various events such as Change, Click, MouseMove and many more that will be listed in the Properties dropdown list in the code window for the TextBox control. We will look into a few of them as given below.

  • The code entered in the Change event fires when there is a change in the contents of the TextBox

  • The Click event fires when the TextBox control is clicked.

  • The MouseMove event fires when the mouse is moved over the TextBox

As explained above, several events are associated with different controls and forms, some of the events being common to most of them and few being specific to each control.

The above article doesn't give you any guarantee and the sole purpose of this article is to share my learning in the way I understood. Any comments to refine this article are welcome with great pleasure. Please report any breaking link by commenting below.

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