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Notice that the generated name of the event handler is similar to the name given to event procedures in classic versions of Visual Basic However, the event handler name no longer has any intrinsic meaning It is just the method (subroutine) name You could modify the preceding event handler as follows:
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Private Sub ProcessEnter(ByVal sender As Object, _ ByVal e As SystemEventArgs) Handles NameTextBoxEnter End Sub
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It is the Handles keyword that de nes the control and event that the handler processes The ProcessEnter method handles the Enter event for the textbox named NameTextBox because that is the control and event that are de ned in the Handles clause Once you create the event handler with the correct signature, the event handler appears in the Events view of the Properties window for the control, as shown in Figure 45
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Figure 45 The Events view of the Properties window lists all the events that are implemented for the control, along with the name of the method de ned as the event handler
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NOTE: If you add the event handler using AddHandler, as described later in this section, it is not listed for the event in the Properties window
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The parameters for all control event handlers are similar The rst parameter de nes the object, usually the control, that generated the event:
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ByVal sender As Object
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The parameter is declared As Object to de ne a general sender object This allows the same event handler to be used for many different types of controls To access the sender object s properties or methods, you need to cast the sender variable from the general Object data type to a speci c control type Casting is translating a variable that is de ned to be of one type to another type Visual Basic and the NET Framework provide several different ways to cast variables from one type to another You can use the Visual Basic conversion functions such as CStr, CInt, and CType If you are converting to a string, you can use the NET Framework ToString method If you are converting from a generalized type to a more speci c type, you can use the DirectCast or TryCast keywords
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NOTE: DirectCast and TryCast provide somewhat better performance than CType when you re converting from the Object data type to a more specific data type
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DirectCast casts a variable from a de ned type to a more specialized type in the same object hierarchy TryCast is similar to DirectCast, but it doesn t fail if the operation is unsuccessful Instead, it returns Nothing You can use the DirectCast and TryCast keywords only when casting from a general type to a more speci c type, such as from type Object to type Control or, even more speci cally, to type TextBox The cast type must be in the same object hierarchy as the original type So you can use these keywords to cast from a variable declared as a general Object data type to a Control data type, because a control is an object, but you cannot use them to cast from a String data type to an Integer data type, because an integer is not a string Cast a variable from an object type to a more speci c type as follows:
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Private Sub ProcessEnter(ByVal sender As Object, _ ByVal e As SystemEventArgs) Handles NameTextBoxEnter
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Dim tb As TextBox tb = DirectCast(sender, TextBox) End Sub
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This code casts the sender object variable from a general Object data type to the more speci c TextBox type The DirectCast method is used here instead of the TryCast method because this is an event handler for the TextBox control, so there is little possibility that the cast could fail The second parameter of the event handler de nes the event arguments It normally has this form:
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