multiple criteria for an if statement - vb.net

Question

I have a form with 3 text boxes, txt_customeracc, txt_customername, txt_customercontact
These 3 text boxes are optional and by default the text boxes will have "N/A" displaying on form load, but if a user enters information into one of them I want them to enter the information in the other two boxes also.
The code I am using is below
If txt_customername.Text <> "" Or txt_customername.Text <> "N/A" Or
txt_customercontact.Text <> "" Or txt_customercontact.Text <> "N/A" And
txt_customeracc.Text = "" Or txt_customeracc.Text = "N/A"
Then error1 += vbNewLine & "Please enter a correct Customer Account Number"
So from the above code I am expecting that if a user enters information in either the txt_customername or the txt_customercontact text boxes but not in the txt_customeracc box the warning should then appear but currently the warning message is displaying regardless of whether information is or isn't entered in any of the boxes. Can anyone tell me what I am doing wrong?

Answer 1

What is operator precedence ?
Your main problem here is that you have an issue with operator precedence. What is that ?
It is exactly the same issue as when doing calcuations, multiplication comes first, then comes addition. Well in VB .NET, And operator comes before Or, so what you have written in your code is evaluated as follow :
If txt_customername.Text <> "" Or
txt_customername.Text <> "N/A" Or
txt_customercontact.Text <> "" Or
(txt_customercontact.Text <> "N/A" And txt_customeracc.Text = "") Or
txt_customeracc.Text = "N/A"
Then
error1 += vbNewLine & "Please enter a correct Customer Account Number"
End If
Since this is not really what you want, let's build this together :
if customername OR customercontact is filled up
AND
customeracc is empty
That would give us :
if (
(txt_customername.Text <> "" Or txt_customername.Text <> "N/A") 'CustomerName is filled up
Or
(txt_customercontact.Text <> "" Or txt_customercontact.Text <> "N/A") 'Customer Contact is filled up
)
And
(txt_customeracc.Text = "" Or txt_customeracc.Text = "N/A") 'Customer account is empty
Then
'Do whatever
End If
Make it better, call a function
Another problem here is readability, this code may have errors because it's hard to read, so hard to debug.
What we can do is build a function that will check if a textbox is empty :
Private Function IsEmpty(Tb As Textbox) As Boolean
'Here we return true if tb.Text is empty or contains "N/A"
Return Tb.Text = "" Or Tb.Text = "N/A"
End Function
So that would make this a bit more readable :
if (Not IsEmpty(txt_customername) Or Not IsEmpty(txt_customercontact)) 'CustomerName or Customer Contact is filled up
And IsEmpty(txt_customeracc) 'Customer account is empty
Then
'Do whatever
End If
Make it better (2), Compare the strings
As stated by zaggler in his comment, here we don't use String Comparison. What if a user starts typing, then decides to put it back to N/A and writes it lowercase ("n/a") ? Well, we will make a mistake, believing that he did fill up the Textbox and you will end up searching for user "n/a" in your database, which is not a very good idea...
So let's compare the String, make our function even better :
Private Function IsEmpty(Tb As Textbox) As Boolean
'Here we return true if tb.Text is empty or contains "N/A" (or "n/a")
Return Tb.Text = "" Or (String.Compare(Tb.Text, "N/A", True) = 0)
End Function
Note
You can see here the advantage of functions. I wrote it because I didn't want to change to String.Compare() six times... Whenever you have the same code twice, it should be a function...

Answer 2

If you want to give the user a proper error message telling him what he has missed to fill out you have to split the if statement into several parts.
First check if all text boxes contain any valid data.
If not you can skip further checks directly.
If one textbox contains data check each and set error accordingly.
If (txt_customername.Text = "" OrElse txt_customername.Text = "N/A") AndAlso
(txt_customercontact.Text = "" OrElse txt_customercontact.Text = "N/A") AndAlso
(txt_customeracc.Text = "" OrElse txt_customeracc.Text = "N/A") Then
'No Error exit sub
Exit Sub
End If
'This part is only reached if one textbox contains data
If (txt_customername.Text = "" OrElse txt_customername.Text = "N/A") Then
error1 += vbNewLine & "Please enter a correct Customer Name"
End If
If (txt_customercontact.Text = "" OrElse txt_customercontact.Text = "N/A") Then
error1 += vbNewLine & "Please enter a correct Customer Contact"
End If
If (txt_customeracc.Text = "" OrElse txt_customeracc.Text = "N/A") Then
error1 += vbNewLine & "Please enter a correct Customer Account Number"
End If
As you can see I also recommend using short circuit OrElse and AndAlso which gives a littttttttle performance.

Answer 3

You could count the number of filled fields.
Dim numberFilled As Integer = 0
If txt_customername.Text <> "" And txt_customername.Text <> "N/A" Then
numberFilled += 1
End If
If txt_customercontact.Text <> "" And txt_customercontact.Text <> "N/A" Then
numberFilled += 1
End If
If txt_customeracc.Text <> "" And txt_customeracc.Text <> "N/A" Then
numberFilled += 1
End If
If numberFilled = 1 Or numberFilled = 2 Then
error1 += vbNewLine & "Please enter a correct Customer Account Number"
End If
Personally I would have a function IsValueEmpty that would check:
Function IsValueEmpty(ByVal value As String) As Boolean
If String.IsNullOrEmpty(value) Or value = "N/A" Then
Return True
End If
Return False
End Function
Could also Trim.

Answer 4

You could have the relevant message part stored in the Tag property of each textbox and use Linq :
Dim customerTextBoxes = {txt_customeracc, txt_customername, txt_customercontact}
Dim messages = Aggregate customerTextBox In customerTextBoxes
Where customerTextBox.Text = "" OrElse customerTextBox.Text = "N/A"
Select $"Please enter a correct {customerTextBox.Tag}")
Into ToArray
Then just check it's length against the initial one and if they're not equal aggregate the message for display
If customerTextBoxes.Length <> messages.Length Then error1 = String.Join(Environment.NewLine, messages)