Validating a string - vb.net

Question

I am trying to write a program where the user inputs a telephone number and a message is displayed if it contains a letter.
Module Module1
Sub Main()
Dim TelNumber As String
Dim Character As String
TelNumber = Console.ReadLine()
For Number = 0 To TelNumber.Length - 1
Character = TelNumber.Substring(Number)
If Integer.TryParse(Character, 0) <> 0 Then
Console.WriteLine("It cannot contain a letter")
Exit For
End If
Next
Console.ReadLine()
End Sub
End Module
However with this code even it only works properly if the string contains less than 11 charcaters, after that even if it does not contain any letters it still displays the message. How do I fix this? Also I don't understand what the second parameter of the Integer.TryParse function represents?

Answer 1

TelNumber.Substring(Number) does not just the Numberth character of the string.
It returns the string with Number characters stripped off its beginning.
Thus, in the first step of the loop, TelNumber.Substring(0) returns the whole string.
Then, Integer.TryParse() fails with integer overflow with long string.
Hint: simple string validation is a task for regular expressions.
With regular expressions it will also be very easy to extend tel.number format to something like [phone] or 12-34-56.
Imports System.Text.RegularExpressions
Module Module1
Sub Main()
Dim telNumber As String = Console.ReadLine()
Dim telValidationRegex As Regex = New Regex("^\d+$")
If NOT telValidationRegex.Match(telNumber).Success Then
Console.WriteLine("Wrong telephone number format")
End If
Console.ReadLine()
End Sub
End Module

Answer 2

I don't have a compiler handy, but I can tell you what I think this should look like. First let's look at Int32.TryParse's documentation
result
Type: System.Int32
When this method returns, contains the 32-bit signed integer value equivalent of the number contained in s, if the conversion succeeded, or zero if the conversion failed. The conversion fails if the s parameter is null or String.Empty, is not of the correct format, or represents a number less than MinValue or greater than MaxValue. This parameter is passed uninitialized.
So, the second parameter is supposed to be an integer variable that contains the result of trying to parse the string.
Then, let's look at substring
startIndex
Type: System.Int32
The zero-based starting character position of a substring in this instance.
So what you're doing isn't looking at each character and trying to convert it to an integer. It's looking at the whole number, and trying to convert that to an integer. Then the whole number, except for the first character, and trying to convert that to a number. And so on.
I suspect you'll find that Int32 can only store numbers about 10 digits long (2^32, or 4294967296)
What you want to do is look at each character, something like this.
Module Module1
//Warning, did not try to compile
Sub Main()
Dim TelNumber As String
Dim Character As String
TelNumber = Console.ReadLine()
For Number = 0 To TelNumber.Length - 1
Character = TelNumbert(Number)
Dim OutputNumber as Integer
If Not Integer.TryParse(Character, OutputNumber) Then
Console.WriteLine("It cannot contain a letter")
Exit For
End If
Next
Console.ReadLine()
End Sub
End Module