qais qais - 6 months ago 42
Vb.net Question

GoTo statements and alternatives in VB.NET

I've posted a code snippet on another forum asking for help and people pointed out to me that using

GoTo
statements is very bad programming practice. I'm wondering: why is it bad?

What alternatives to
GoTo
are there to use in VB.NET that would be considered generally more of a better practice?

Consider this snippet below where the user has to input their date of birth. If the month/date/year are invalid or unrealistic, I'd like to loop back and ask the user again. (I'm using if statements to check the integer's size... if there's a better way to do this, I'd appreciate if you could tell me that also :D)

retryday:
Console.WriteLine("Please enter the day you were born : ")
day = Console.ReadLine
If day > 31 Or day < 1 Then
Console.WriteLine("Please enter a valid day")
GoTo retryday
End If

Answer

I'm going to differ from everyone else and say that GOTOs themselves are not all the evil. The evil comes from the misuse of GOTO.

In general, there is almost always better solutions than using a GOTO, but there really are times when GOTO is the proper way to do it.

That being said, you are a beginner, so you shouldn't be allowed to judge if GOTO is proper or not (because it hardly ever is) for a few more years.

I would write your code like this (my VB is a bit rusty...):

Dim valid As Boolean = False

While Not valid
    Console.WriteLine("Please enter the day you were born: ")

    Dim day As String

    day = Console.ReadLine

    If day > 31 Or day < 1 Then
        Console.WriteLine("Please enter a valid day.")
    Else
        valid = True
    End If
End While

If you take your GOTO code and look at it, how would someone first approach your code? "Hmm.. retryday? What does this do? When does this happen? Oh, so we goto that label if the day is out of range. Ok, so we want to loop until the date is considered to be valid and in range".

Whereas if you look at mine:

"Oh, we want to keep doing this until it's Valid. It is valid when the date is within range."

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