How to check if input comes null/empty - vb.net

Question

I need to check if an input comes null/empty.
If input <= 100 And input > 0 And Not input = "" Then
subjectsInt.Add(subjects(i), input)
check = True
End If
I know in that code I am checking it as String but I already did Not input = Nothing and Not input = 0 and I am getting an error:
Run-time exception (line -1): Conversion from string "" to type 'Integer' is not valid.
Stack Trace:
[System.FormatException: Input string was not in a correct format.]
[System.InvalidCastException: Conversion from string "" to type 'Integer' is not valid.]
Any suggestions?
EDIT
Here is the code in case you want to take a look: https://dotnetfiddle.net/0QvoAo

Answer 1

Well, I have looked at your code and there are many problems, all caused by the freely approach to Data Types that VB.NET allows when you have Option Strict set to Off. In this context VB.NET allows you to assign the return value of Console.ReadLine to an Integer trying to help you adding an implicit conversion of the input. Of course if the user types ABCD this implicit conversion has no other way to inform you than triggering an Exception. So I really, really recommend you to use Option Strict set to On (MSDN Option Strict)
Now with Option Strict On your code has a lot of errors, I have rewritten your code and then explain in comments the changes.
Option Strict On
Imports System
Imports System.Collections.Generic
Public Module Module1
Public Sub Main()
Console.WriteLine("Hello World")
' I use a List of strings instead of an untyped array list
Dim subjects As New List(Of String)() From
{
"Math",
"English",
"German"
}
Dim subjectsInt As New System.Collections.Generic.Dictionary(Of String, Integer)
Dim i, input As Integer
Dim check As Boolean
For i = 0 To subjects.Count - 1
check = False
Do
Console.WriteLine(subjects(i) & ": ")
' The input is passed to a string
Dim temp = Console.ReadLine()
' Check if this string is really a number
If Int32.TryParse(temp, input) Then
If input <= 100 And input > 0 Then
subjectsInt.Add(subjects(i), input)
check = True
End If
End If
Loop While check = False
Next
For i = 0 To subjects.Count - 1
Console.WriteLine(subjects(i) & ": " & subjectsInt(subjects(i)))
Next
Console.ReadLine()
End Sub
End Module

Answer 2

The error is telling you that the input is an integer, you can't compare an integer with a string which is precisely what's happening.
Even if the string contains only digit, the type system doesn't convert it inferring from the operator.
In your example, input = "" input can never be "". You should change the type of input to a string and then check before converting it to an integer.
This would be a solution:
Dim integerInput As Integer
Dim stringInput = Console.ReadLine()
If Integer.TryParse(stringInput, integerInput) AndAlso integerInput <= 100 And integerInput > 0 Then
To use < > = with an integer you need to convert the string:
If Integer.Parse(input) <= 100 And Integer.Parse(input) > 0 And Not input = "" Then
Integer.Parse will do the job.
Obviously, if your input is a string and it's not guaranteed that it is going to contain just digit you need either to do some check before or use Integer.TryParse.
The closest you can get to a null check with a value type is EqualityComparer(Of Integer).Default.Equals(id, Nothing); unless you start using Nullable, but it would be too late as the code will fail on the input = Console.ReadLine() when you input a non-digit string.