Q:

Why do I get stack overflow BEFORE all the possible combinations have been reached?

So.
I am making a bruteforcer in visual basic.
It has a charset as shown below:
Dim charset as string
charset = "abcdefghijklmnopqrstuvwxyz1234567890."

There is 37 different chars right? The program is made to search for all the different combinations  made with this charset, with a maximum of 3 different letters. For example
This is a combination that can be made: ac6 
So since there is 37 letters and 3 slots the possible combinations are 37^3
But I wanted my program not to try the same combination twice.
So it saves every single combination tried in this location (Desktop)
Dim filex As System.IO.StreamWriter
            filex = My.Computer.FileSystem.OpenTextFileWriter("c:\Users\" + Environment.UserName + "\desktop\alreadytested.txt", True)
            filex.WriteLine(combination)
            filex.Close()

And, at the start of the Sub that checks for new combinations, I have this
text = File.ReadAllText("c:\Users\" + Environment.UserName + "\desktop\alreadytested.txt")
index = text.IndexOf(combination) 'checks if it has been generated already
   If index >= 0 Then
       keyword() 'The sub
   End If

But after some combinations (in this case the max 37^3 ~= 50.000 and I the program tried around 5200 times) I get this error
An unhandled exception of type 'System.StackOverflowException' occurred in mscorlib.dll
And this error points in this line of code
filex = My.Computer.FileSystem.OpenTextFileWriter("c:\Users\" + Environment.UserName + "\desktop\alreadytested.txt", True)

But why? at 5200 there is still 44800 possible random combinations, why do I get an overflow?
It would make sense if I got it when I had 50000 combinations out of 50000 possible tested, but now I have 10% only, so why do I get an overflow?

A:

You keep on recursively calling the keyword() method. Every time you call a method its return address and possibly its arguments is/are added to the call stack. The callstack can only take a certain amount of calls before it overflows, and for your computer and that specific method of yours, that amount seems to be about 5200.
You should switch to using for example a While-loop instead, and whenever you want to block the rest of the execution and go back to the beginning of the loop you'd just call Continue While.

A little side note is also that you shouldn't open and close the file every time you read/write to it. Store the contents of the file in a long string (or even better, in a HashSet(Of T)) instead and check that every time you need to, then at the end of the loop you may write all the contents to a file.
If you still wish to write to the file during the process then do so. But instead open a stream before your loop which you keep writing to until the loop is finished, then close the stream.

