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What is the way to deal with VB.NET Override, many novices are not very clear about this, in order to help you solve this problem, the following editor will explain for you in detail, people with this need can come to learn, I hope you can gain something.

After learning the programming language VB.NET, we will slowly find that the hidden knowledge in this language is very wide, and there are many functions worthy of our in-depth study. For example, VB.NET Override is one of the more difficult knowledge points to master. In the inheritance of a class or control, a child class or child control encapsulates all functions of the parent class, including event handlers.

Inherit event handler

When a component is inherited, all its members are encapsulated into subclasses. An event handler is a method that responds to specific events, is a member of a component, and is therefore inherited. Look at a typical event handler:

Private Sub Button1_Click (ByVal

Sender As System.Object

ByVal e as _

System.EventArgs) Handles

Button1.Click

Static Counter as Integer = 0

Counter + = 1

MessageBox.Show ("This button"

Has been clicked "& _

Counter.ToString () & "times.")

End Sub

We see that the Handles clause at the end of the declaration section associates the handler to a specific event. Specifically, the above code will run when the Button1.Click event occurs. This is the typical syntax used to define event handlers.

To implement VB.NET Override, you must use the Overridable keyword and redefine its visibility to Protected, Protected Friend, or Public. The following demonstrates how to override event handlers:

Protected Overridable Sub

Button1_Click (ByVal sender

As System.Object, _

ByVal e as System.EventArgs)

Handles Button1.Click

Static Counter as Integer = 0

Counter + = 1

MessageBox.Show ("This button"

Has been clicked "& _

Counter.ToString () & "times.")

End Sub

Inherit event handlers in a component

Although the VB.NET Override of the event handler is more or less the same as the override of other methods, one thing has to be mentioned: the Handles clause must be removed from the override event handler.

How to: event handlers for override components

1) add the Overrides keyword to the declaration section of the event handler

Please note: do not attach the Handles clause. Because the handler of the parent class is already associated with a specific event, the subclass will inherit all of this. In other words, the event associated with the parent class can activate the handler of the subclass, so the Handles clause here is redundant.

Here is an example of how to override the aforementioned event handler:

Protected Overrides Sub

Button1_Click (ByVal sender

As System.Object, _

ByVal e as System.EventArgs)

Static Counter as Integer = 0

Counter + = 1

MessageBox.Show ("This inherited"

Button has been clicked "& _

Counter.ToString () & "times.")

End Sub

2) Why not need the Handles clause

As mentioned earlier, our failure to use the Handles clause associated with events is not an omission, but is determined by the event handling mechanism of the .NET Framework. The Handles clause in the parent class associates the event handler to a specific event, which is inherited by the subclass. Therefore, even if there is no Handles clause in the subclass, the event associated with the parent class can activate the event handler of the subclass. If you add the Handles clause at this time, the handler will be associated with the event again. The consequence of repeated associations is that the handler will be activated twice in a row. For example:

'incorrect code

Protected Overrides Sub

Button1_Click (ByVal sender As

System.Object, _

ByVal e as System.EventArgs) Handles

Button1.Click

Static Counter as Integer = 0

The Handles clause is used in this example

So every time the button Button1 is clicked

'The variable Counter will be added twice

Counter + = 1

'The message box will pop up twice.

'The content displayed will also run counter to the design intention

MessageBox.Show ("This inherited"

Button has been clicked "& _

Counter.ToString () & "times.")

End Sub

Conclusion

In Visual Basic .NET, the VB.NET Override of event handlers can result in bug that is difficult to debug. For example, extra care should be taken when using the Handles clause to associate event handlers to avoid repeated associations.

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