Validation
"Programming today is a race between software
engineers striving to build bigger and better idiot-proof
programs and the Universe trying to produce bigger
and better idiots. So far, the Universe is winning.“
Rich Cook
Robust and Responsive
 Robust means that a user may type any data they like
into the program and it won’t crash
 Responsive means that the program will tell the user
what they have done wrong and what they should do
to correct the error
What is wrong with the following?
Dim FirstName as Integer
FirstName=”Fred”
What is wrong with the following?
Dim Age as Integer
Age = txtAge.Text
IsNumeric
‘declare a variable to store the age
Dim Age as Integer
‘test the data typed in the text box to see if it is a number
If IsNumeric(txtAge.Text)=True Then
‘if it is ok read in the value
Age = txtAge.Text
Else
‘if not ok then show an error
lblError.Text = "The age is not valid"
End If
IsDate
‘declare a variable to store the date of birth
Dim DOB as Date
‘test the data typed in the text box to see if it is a date
If IsDate(txtDOB.Text)=True Then
‘if it is ok read in the value
DOB = txtDOB.Text
Else
‘if not ok then show an error
lblError.Text = "The date of birth is not a valid date"
End If
Testing for a blank field (=“”)
‘declare a variable to store first name
Dim FirstName as String
‘test the data typed in the text box to see if it isn’t blank
If txtFirstName.Text<>"" Then
‘if it is ok read in the value
FirstName = txtFirstName.Text
Else
‘if not ok then show an error
lblError.Text = "You must enter your first name."
End If
Testing Number of Characters (Len)
‘declare a variable to store first name
Dim FirstName as String
‘test the data typed in the text box is not over 20 characters
If Len(txtFirstName.Text)<=20 Then
‘if it is ok read in the value
FirstName = txtFirstName.Text
Else
‘if not ok then show an error
lblError.Text = "First name must not exceed 20 letters."
End If
Validating an Email Address (InStr)
‘declare a variable to store the email address
Dim EMail as String
‘test the data typed in the text box has an @ symbol
If InStr(txtEMail.Text, "@") >0 Then
‘if it is ok read in the value
EMail = txtEMail.Text
Else
‘if not ok then show an error
lblError.Text = "Not a valid email address."
End If
Question – What is the problem with the above code?
The problem is…
that the following are all valid email addresses..
fred@
@fred@
@
@@
@@@@@@@@
An Email Address Must Have…
 “@” symbol e.g. @hotmail.com
.
 “ ” symbol e.g. hotmail.com
 At least 5 characters (?)
(What is wrong with the following if we use fred@nothing ?)
Dim EMail as String
‘test the data typed in the text box has an @ symbol
If InStr(txtEMail.Text, "@") >0 Then
EMail = txtEMail.Text
Else
lblError.Text = "Not a valid email address."
End If
‘test the address is long enough
If Len(txtEMail.Text) >5 Then
EMail = txtEMail.Text
Else
lblError.Text = "The address is too short."
End If
‘test the data typed in the text box has an . symbol
If InStr(txtEMail.Text, ".") >0 Then
EMail = txtEMail.Text
Else
lblError.Text = "Not a valid email address."
End If
Better???
‘declare a variable to store the email address
Dim EMail as String
‘test the data typed in the text box has an @ symbol
If InStr(txtEMail.Text, "@") >0 Then
‘test the address is long enough
If Len(txtEMail.Text) >5 Then
‘test the data typed in the text box has an . symbol
If InStr(txtEMail.Text, ".") >0 Then
‘if it is ok read in the value
EMail = txtEMail.Text
Else
lblError.Text = "Not a valid email address."
End If
Else
lblError.Text = "The address is too short."
End If
Else
lblError.Text = "Not a valid email address no @ symbol."
End If
Or….
‘declare a variable to store the email address
Dim EMail as String
‘clear the error message label
lblError.Text = ""
‘test the data typed in the text box has an @ symbol
If InStr(txtEMail.Text, "@") = 0 Then
lblError.Text = "No @ symbol."
End If
‘test the address is long enough
If Len(txtEMail.Text) <=5 Then
lblError.Text = lblError.Text & " The address is too short."
End If
‘test the data typed in the text box has a . symbol
If InStr(txtEMail.Text, ".") = 0 Then
lblError.Text = lblError.Text & "No dot."
End If
If lblError.Text = "" Then
Email = txtEmail.Text
End IF
Validation in Action
 We shall look at the validation for the save button on “your
details”
Questions
1. What is wrong with the following validation?
Dim Age as Integer
Age = txtAge.Text
If IsNumeric(Age) = True Then
If Age > 18 Then
lblMessage.Text = “You are old enough”
End If
End If
Questions
2. Write code that validates if the text in txtName is less
than 40 characters. If the length of the name is OK
then a message should appear welcoming the person.
3. Write code that validates the text in txtPostCode to
ensure that it is not blank.
4. Write code that validates the values in the fields,
txtStartAge and txtEndAge also making sure that the
start age is not greater than or equal to the end age.