Lesson 7 - Sanitizing user input in VB.NET
In the previous exercise, Solved tasks for Visual Basic .NET lesson 6, we've practiced our knowledge from previous lessons.
Lesson highlights
Are you looking for a quick reference on sanitizing user input in VB.NET instead of a thorough-full lesson? Here it is:
Using the TryParse()
method to handle
invalid user inputs and a While
loop to
keep the user entering:
Console.WriteLine("Enter a number:") Dim a As Double While Not Double.TryParse(Console.ReadLine(), a) Console.WriteLine("Invalid entry, please try again:") End While
Using ReadKey()
instead of
ReadLine()
when interested in one character only and the
Else
branch of a Select Case
to handle invalid
inputs:
Console.WriteLine("Do you like VB.NET?:") Console.WriteLine("1 - yes") Console.WriteLine("2 - maybe") Console.WriteLine("3 - no") Dim choice As Char = Console.ReadKey().KeyChar Console.WriteLine() Select Case choice Case "1" Console.WriteLine("Me too!") Case "2" Console.WriteLine("Come on, it's a nice language!") Case "3" Console.WriteLine("Maybe you should study harder") Case Else Console.WriteLine("Invalid option!") End Select Console.ReadKey()
Would you like to learn more? A complete lesson on this topic follows.
In the previous lesson, Solved tasks for Visual Basic .NET lesson 6, we introduced loops. Today's tutorial is going to be a little more relaxing because we're going to finish our calculator in Visual Basic .NET. We won't need it anymore after this, and it would be nice to finish it. You might already know that it lacks user input sanitation, which is what we're going to do today.
Let's bring up our calculator code:
Dim goOn As String = "yes" While goOn = "yes" Console.WriteLine("Enter the first number:") Dim a As Double = Console.ReadLine() Console.WriteLine("Enter the second number:") Dim b As Double = Console.ReadLine() Console.WriteLine("Choose one of the following operations:") Console.WriteLine("1 - addition") Console.WriteLine("2 - subtraction") Console.WriteLine("3 - multiplication") Console.WriteLine("4 - division") Dim choice As Integer = Console.ReadLine() Dim result As Double = 0 Select Case choice Case 1 result = a + b Case 2 result = a - b Case 3 result = a * b Case 4 result = a / b End Select If choice > 0 And choice < 5 Then Console.WriteLine("Result: {0}", result) Else Console.WriteLine("Invalid choice") End If Console.WriteLine("Would you like to make another calculation? [yes/no]") goOn = Console.ReadLine() End While Console.WriteLine("Thank you for using our calculator. Press any key to end the program.") Console.ReadKey()
I had mentioned earlier that we should always sanitize user inputs. Let me
tell you the secret to making successful and popular applications. It's very
simple: You treat your users like total fools The sillier you expect the user to
be, the more successful your applications will be. If the user enters
"yes "
(yes + space) rather than simply "yes"
,
or he/she enters "Yes"
(with a capital "Y"), the program
would terminate. Which technically might not be due to user's silliness, more so
because the user mistyped it. They could, however, enter something totally
ridiculous, like: "maybe"
.
Either way, that's not the biggest problem in our program. When the user doesn't enter a number, but some nonsense instead, the whole program crashes with an error. Let's fix these two problems.
To validate the input before parsing it, we can use the
TryParse()
method instead of Parse()
. The method
returns True
/False
depending on whether the parsing
succeeded or not. If you're asking how we get the parsed value from the method,
the answer is that it'll be stored in a variable which we pass as the second
parameter. The value of that variable will be affected. Let's now look at the
sanitized retrieving of the first number, the parsing of the second number will
be obviously analogical, so we can just copy it. Ideally, we should have created
a method for it, so we wouldn't write the same code twice, but now is not the
time to deal with all of that. We'll learn how to declare methods in the
object-oriented programming course.
Console.WriteLine("Enter the first number:") Dim a As Double While Not Double.TryParse(Console.ReadLine(), a) Console.WriteLine("Invalid entry, please try again:") End While
There is nothing difficult about the code shown above. First, we ask the user
to enter the number and we declare the variable a
. Then we insert
TryParse()
in the condition of a While
loop and negate
this condition with the Not
operator. So while the method returns
False
, the loop will keep going and asking the user to enter
another value. The entered text from the console will be parsed into a variable
and the method will return True
. If parsing fails, it will return
False
.
Now, let's look at the operational selection and continuation parts. We read
both inputs as string
s even though it's not quite appropriate to do
so. It makes sense to read the numbers as string
s since they may be
longer that one character, so they must be submitted by pressing
Enter during the selection of operations (1-4). Nonetheless, it's
enough to read just a single character and we don't need to submit it by
pressing enter. We read a single character using the
Console.ReadKey()
method, which we've already covered. To get the
result as Char
(character), we need to use the KeyChar
property.
Dim choice As Char = Console.ReadKey().KeyChar Dim result As Double = 0 Dim validChoice As Boolean = True Select Case choice Case "1" result = a + b Case "2" result = a - b Case "3" result = a * b Case "4" result = a / b Case Else validChoice = False End Select If validChoice Then Console.WriteLine("Result: {0}", result) Else Console.WriteLine("Invalid choice") End If
We store the entered character as a Char
into the variable
choice
. Because the range of characters can't be easily tested with
conditions as with integer
s with our current knowledge, but we'll
check it in another way. We prepare a Boolean
variable
validChoice
, which is set to True
(which we assume to
be correct). The Select Case
remains the same, we only put numbers
in quotes because they're characters now. We add the Case Else
,
which will set our validChoice
variable to False
in
case that some unspecified value was entered. Then, there is nothing easier than
to test that variable. Try it, the program is much more intuitive now.
Finally, we need to modify the continuing prompt. We'll enter the Y/N
char
acters and we'll make the input case-insensitive and respond to
invalid values. We'll use the Select Case
again and change our
variable, goOn
, to the Boolean
datatype. It is
unnecessary to describe this code, all that's worth mentioning is the
Console.ReadKey().KeyChar.ToString().ToLower()
i.e. method
chaining, which reads a character from the console and returns it as a lowercase
String
.
Since this is a bigger piece of code, we'll use comments. We write them with single quotation marks. It is information for the programmer which the compiler ignores.
Dim goOn As String = "yes" While goOn = "yes" ' reading numbers Console.WriteLine("Enter the first number:") Dim a As Double While Not Double.TryParse(Console.ReadLine(), a) Console.WriteLine("Invalid entry, please try again:") End While Console.WriteLine("Enter the second number:") Dim b As Double While Not Double.TryParse(Console.ReadLine(), b) Console.WriteLine("Invalid entry, please try again:") End While ' operation choice and calculation Console.WriteLine("Choose one of the following operations:") Console.WriteLine("1 - addition") Console.WriteLine("2 - subtraction") Console.WriteLine("3 - multiplication") Console.WriteLine("4 - division") Dim choice As Char = Console.ReadKey().KeyChar Dim result As Double = 0 Dim validChoice As Boolean = True Select Case choice Case "1" result = a + b Case "2" result = a - b Case "3" result = a * b Case "4" result = a / b Case Else validChoice = False End Select If validChoice Then Console.WriteLine("Result: {0}", result) Else Console.WriteLine("Invalid choice") End If Console.WriteLine("Would you like to make another calculation? [yes/no]") ' request to continue validChoice = false While Not validChoice Select Case Console.ReadKey().KeyChar.ToString().ToLower() Case "y" goOn = True validChoice = True Case "n" goOn = False validChoice = True Case Else Console.WriteLine("Invalid option, please enter y/n") End Select End While End While Console.WriteLine("Thank you for using our calculator. Press any key to end the program.") Console.ReadKey()
Console application
Welcome to our calculator
Enter the first number:
number
Invalid entry, please try again:
13
Enter the second number:
22
Choose one of the following operations:
1 - addition
2 - subtraction
3 - multiplication
4 - division
3
Result: 286
Would you like to make another calculation? [yes/no]
h
Invalid choice, please enter y/n
Congratulations, you've just created your first foolproof program The code became a little more
complicated, but it's worth it in the end. In the future, we may refactor it and
split it up into separate methods. We'll say that our calculator is done for now
(for this course, anyway). We could maybe add some more mathematical functions,
but we'll get to that later in the course.
In the next lesson, Arrays in VB.NET, we'll dive into new constructs. Arrays and
advanced work with strings await our arrival. Then, we'll finish the constructs
in this course. We're approaching end