Input
Validation: Good portions of program errors come from unexpected user input
or responses. “How do I prevent a user from entering a letter into a Text Box
intended for numbers?” In short, input validation really involves a talented
programmer with enough foresight to prevent errors before they happen.
Validate
Event: Most Visual Basic controls have a validate event that occurs
right before the control loses focus. However, this event is not always
triggered by default. To ensure it does, you can set the CausesValidation
property of the control to true.
Private Sub Form_Load()
Text1.CausesValidation =
True
End Sub
The
validate event of the Text Box takes the Boolean variable Cancel as an argument:
Private Sub
Text1_Validate(Cancel As Boolean)
If Text1.Text = “A” Then
Cancel = False
Else
Cancel = True
End If
End Sub
If
the user enters the letter A into the Text Box, I set the Cancel
parameter to false. This lets the user move on to the next control. However, if
the user enters anything else, I set the cancel parameter to true, essentially
preventing the user from going anywhere.
Checking
Data Types: Sometimes, preventing input errors can be as easy as determining
whether a user has entered a number or a string. You might want the user to
enter his or her name, or maybe you are looking for a number such as an age.
Either way, Visual Basic provides the IsNumeric function for testing such
scenarios.
The
IsNumeric function takes a variable or expression as a parameter and returns a
Boolean value of true if the variable or expression is a number and false if it
is not. You can use the IsNumeric function in conjunction with the validate
event for checking data types:
Private Sub
Text1_Validate(Cancel As Boolean)
If IsNumeric(Text1.Text) =
True Then
Cancel = False
Else
Cancel = True
End If
End Sub
Testing
a Range of Values: You might find at times that testing a value for a particular
data type is not enough to prevent input errors. Sometimes, it is necessary to
check for a range of values. For example, you might want to prompt a user to
enter a number from 1 to 100. Or maybe you want a person to choose a letter
from a to z
Private Sub
Text1_Validate(Cancel As Boolean)
If IsNumeric(Text1.Text) =
True Then
If Val(Text1.Text) >= 1
And Val(Text1.Text) <= 100 Then
Cancel = False
Else
Cancel = True
End If
Else
Cancel = True
End If
End Sub
