How can I check string in textbox as integer in Visual Basic?

Question

I have 3 textboxes (day, month, year) and I want to check if input is e.g. day has to be from 1 to 31 and so on.

My code is:

If InputDan.Text < "1" Or InputDan > "31" Then Warning.Text = "Not a valid day input." Else Warning.Text = ""

Also I have day and month input limited to 2 characters and year to 4. It works fine with numbers from 10 to 31 and it properly puts an warning message when input is 0 or 32 and on.

Here's the problem...

When I put in numbers from 4 to 9 it puts on a warning message, as I figured out later that program considers empty space after one character input as 0. So if I enter 4 the program will read it as 40, and so on.

Can I solve this problem with converting String input as Int somehow?

Tim Schmelter · Accepted Answer · 2012-11-16T09:25:21.183

You need to parse the numbers to integer before you can compare them, otherwise >"11" will compare them alphabetically and not by their numerical order.

Dim day As Integer
Dim valid As Boolean = Int32.TryParse(InputDan.Text, day)

Now you know if that input was a correct number and you could show a warning if it was not.

I would suggest a different approach to check whether or not the input was a correct day since you must take the number of days in that month into account(also leap years, different calendars etc). So use the current culture's calendar and look if the number of days is correct for the given month in this way:

Dim daysInMonth = CultureInfo.CurrentCulture.Calendar.GetDaysInMonth(year, month)
If day > daysInMonth OrElse day < 1 Then
    ' show warning '
End If

(assuming you have already checked the year and month part with Int32.TryParse)

score 1 · Answer 2 · answered Nov 16 '12 at 08:52

Better than doing this from the code behind, asp.net has already validations here is an example of a textbox that represents the day, and it has to be between 1 and 31:

<asp:TextBox ID="TextBox2" runat="server"></asp:TextBox>

<asp:RangeValidator ID="RangeValidator1" runat="server" 
    ErrorMessage="You have to insert a valid day" ControlToValidate="TextBox2" MaximumValue="31" MinimumValue="1"></asp:RangeValidator>

check it out

I assume that this is winforms. – Tim Schmelter Nov 16 '12 at 09:01 — Tim Schmelter, Nov 16 '12 at 09:01

score 0 · Answer 3 · answered Nov 16 '12 at 08:57

Your problem is, that "9" (the string) IS "larger" than "31". Because sorting is done on the first char, then the second and so on.

    Dim Value As Integer
    ' is there an (integer) NUMBER in the textbox?
    If Integer.TryParse(InputDan.Text, Value) Then
        If Value > 0 AndAlso Value < 31 Then
            ' do something
        Else
            MessageBox.Show("please enter a number!")
        End If
    Else
        MessageBox.Show("please enter a number!")
    End If

With "TryParse" you can test if a String can be converted to an Integer (or Double, Single, whatever implements a TryParse method) and if it can be converted, the value is stored in the second parameter.

score 0 · Answer 4 · answered Nov 16 '12 at 09:32

0

You should use Strict on" to avoid the coding problems - basically you are comparing strings against each other. They do NOT behave like Integers for comparisons.

answered Nov 16 '12 at 09:32

Christian Sauer

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Though `Strict On` wouldn't solve the problem anyway, since from the compiler's view, everything is fine. He just compares two strings but thought to compare "numbers". And unfortunately there is no strict on for the programmer's brain ;) – igrimpe Nov 16 '12 at 10:03

score 0 · Answer 5 · answered Nov 16 '12 at 10:28

Try like this: (Assuming framework above/or 3.5)

If Not IsNumeric(InputDan.Text) OrElse _
   Not Enumerable.Range(1, 31).Contains(CInt(InputDan.Text)) Then
    Warning.Text = "Not a valid day input."
Else
    Warning.Text = ""
End If

It will first validate the input must be a number and then will validate if it lies within range of 1 and 31. I assume Days can not be 1.5 so I called CInt.

OrElse is what we call ShortCircuit. The second condition will not evaluate if the first one failed.

How can I check string in textbox as integer in Visual Basic?

5 Answers5