Visual Basic .substring错误

Question

I'm new to VB and I was having some issues with the following code.

    Dim random As String = "asfdgasfdgasfdgasfd11"
    Dim length As Integer = Nothing

    length = random.Length
    Console.WriteLine(random.Length)
    Console.WriteLine(length)
    Console.WriteLine()
    Console.WriteLine()
    Console.ReadLine()

    If length <= 20 Then
        Console.WriteLine(random.Substring(0, length))
    ElseIf length <= 40 Then
        Console.WriteLine(random.Substring(0, 20))
        Console.WriteLine(random.Substring(20, length))
    End If

    Console.ReadLine()

Error:

" An unhandled exception of type 'System.ArgumentOutOfRangeException' occurred in mscorlib.dll

Additional information: Index and Length must refer to a location within the string "

I think the error is occurring due to ( 20 , length )). I tried to assign length to a variable so the program does not crash unless the trying is a specific number of characters.

I am attempting to have a variable of any given length and if it is greater than 20 characters then only print 20 characters per line.

Answer 1

Additional information: Index and Length must refer to a location within the string

That's the point. In your second WriteLine you ask to print the random string starting from the 20th character (starting index ok, there are 21 characters) but then it ask to print for 21 chars (length = 21).
Yes, startindex + length = 41 and it is out of the string limits

you could try to fix that line with

Console.WriteLine(random.Substring(20, length - 20))

or introduce a while loop that prints 20 characters at time

length = random.Length
Console.WriteLine(random.Length)
Console.WriteLine(length)
Console.WriteLine()
Console.WriteLine()
Console.ReadLine()

Dim curStart = 0 
Dim loopCounter = 0
while(curStart < random.Length)
    Console.WriteLine(random.Substring(curStart, System.Math.Min(20, length - 20 * loopCounter)))
    curStart = curStart + 20
    loopCounter = loopCounter + 1
End While