How do i make a loop that will stop when a scrollbar reaches the bottom

Having done a number of controls over the years, I've always found that the best, and smoothest, way to handle the scrollbar operations is to just to move the contents already painted - if anything you've already painted will be visible after scrilling; and then invalidate the rest of the window.

In other words you have to keep track of what's visible and painted.

The native windows API provides a nice function:

BOOL ScrollDC( HDC hDC,
    int dx,
    int dy,
    const RECT *lprcScroll,
    const RECT *lprcClip,
    HRGN hrgnUpdate,
    LPRECT lprcUpdate
);

That efficiently moves what you've already painted - Documented here[^]

I'd start out by just invalidating the window, and enhance the implementation to reuse what's painted when you've got that part working.

Regards
Espen Harlinn

Vlad, for a programmer, there is not such thing as "crashing". You should use debugger properly and, in order to post a correct Question for a CodeProject, learn how to dump exception information.

Spotting this particular problem based on your posted code is a bit tedious and boring. We could do little better. Let me help you a bit with System.Windows.Forms specific exception handling feature which many developers miss; so it will help you in future as well (and please forgive me if you already know that). Most likely, all your problem unfolds in you UI thread. This is what you have to do first.

In your entry point function (usually called Program.Main), add the following line (assuming "using System.Windows.Forms;"):

[System.STAThread]
static void Main() {
    Application.SetUnhandledExceptionMode(UnhandledExceptionMode.CatchException);
    //...
} //Main

Add some handler of the special Application event fired on exception inside of main loop if Application.Run. This code should be called before you show your main form, as soon as possible, for example, in its constructor:

Application.ThreadException += delegate(
    object sender,
    System.Threading.ThreadExceptionEventArgs eventArgs) {
        System.Windows.Forms.MessageBox.Show(
            string.Format(
                "{0}: \n\n{1}",
                eventArgs.Exception.GetType().Name,
                eventArgs.Exception.Message),
            string.Format(" {0}: Error", Application.ProductName),
            MessageBoxButtons.OK, MessageBoxIcon.Error);
}; //Application.ThreadException

If your Framework version is 3.5 or later, lambda form is more convenient:

Application.ThreadException += (sender, eventArgs) => {
   //...
}; //Application.ThreadException

This will show all your exceptions in UI thread without termination of the application, you also can effectively use debugger break point in this function.

[EDIT]
Additionally or alternatively, handle the events System.AppDomain.UnhandledException and System.AppDomain.FirstChanceException.
My credit to Espen Harlinn for the idea.
[END EDIT]

If it does not provide enough information, add detailed dump: exception stack (first thing to do) and possibly all inner exceptions, recursively. If you want help from CodeProject, save this dump in file and post its content in your Question (please, try to format nicely!).

If your problem is still not resolved, ask follow-up questions, but please not post it as an Answer (important!); use "Improve question" or "Add comment".

By the way, I did not understand why you use P/Invoke, that is, why pure .NET API was not enough?

—SA