C#, WinForms: ListBox.Items.Add generates an OutOfMemoryException, why?

Question

First off, I found the solution to the exception. I'm more curious why it generated the specific exception it did.

In my scenario I'm adding a POCO to a ListBox like so:

myListBox.Items.Add(myPOCO);

This was generating an OutOfMemoryException. The problem was that ToString off the POCO was returning null. I added a string.IsNullOrEmpty check to return a "safe" value when null and the exception went away.

Why does this generate an OutOfMemoryException and not something else (say a NullReferenceException)?

EDIT: Items are added in a for loop.

Full call stack (company-specific references removed) is below. One thing to note -- the list box is empty when this is called.

System.OutOfMemoryException was unhandled
  Message="List box contains too many items."
  Source="System.Windows.Forms"
  StackTrace:
       at System.Windows.Forms.ListBox.NativeAdd(Object item)
       at System.Windows.Forms.ListBox.ObjectCollection.AddInternal(Object item)
       at System.Windows.Forms.ListBox.ObjectCollection.Add(Object item)
       at <FORM>_Load(Object sender, EventArgs e) in <PATH>\<FORM>.cs:line 52
       at System.Windows.Forms.Form.OnLoad(EventArgs e)
       at System.Windows.Forms.Form.OnCreateControl()
       at System.Windows.Forms.Control.CreateControl(Boolean fIgnoreVisible)
       at System.Windows.Forms.Control.CreateControl()
       at System.Windows.Forms.Control.WmShowWindow(Message& m)
       at System.Windows.Forms.Control.WndProc(Message& m)
       at System.Windows.Forms.ScrollableControl.WndProc(Message& m)
       at System.Windows.Forms.ContainerControl.WndProc(Message& m)
       at System.Windows.Forms.Form.WmShowWindow(Message& m)
       at System.Windows.Forms.Form.WndProc(Message& m)
       at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
       at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
       at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
       at System.Windows.Forms.SafeNativeMethods.ShowWindow(HandleRef hWnd, Int32 nCmdShow)
       at System.Windows.Forms.Control.SetVisibleCore(Boolean value)
       at System.Windows.Forms.Form.SetVisibleCore(Boolean value)
       at System.Windows.Forms.Control.set_Visible(Boolean value)
       at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
       at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
       at System.Windows.Forms.Application.RunDialog(Form form)
       at System.Windows.Forms.Form.ShowDialog(IWin32Window owner)
       at System.Windows.Forms.Form.ShowDialog()
       at <APP>.Program.Main() in <PATH>\Program.cs:line 25
       at System.AppDomain._nExecuteAssembly(Assembly assembly, String[] args)
       at System.AppDomain.nExecuteAssembly(Assembly assembly, String[] args)
       at System.Runtime.Hosting.ManifestRunner.Run(Boolean checkAptModel)
       at System.Runtime.Hosting.ManifestRunner.ExecuteAsAssembly()
       at System.Runtime.Hosting.ApplicationActivator.CreateInstance(ActivationContext activationContext, String[] activationCustomData)
       at System.Runtime.Hosting.ApplicationActivator.CreateInstance(ActivationContext activationContext)
       at System.Activator.CreateInstance(ActivationContext activationContext)
       at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssemblyDebugInZone()
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()

Answer 1

This is because of the way System.Windows.Forms.ListBox.NativeAdd method is implemented:

private int NativeAdd(object item)
{
    int num = (int) base.SendMessage(0x180, 0, base.GetItemText(item));
    switch (num)
    {
        case -2:
            throw new OutOfMemoryException();

        case -1:
            throw new OutOfMemoryException(SR.GetString("ListBoxItemOverflow"));
    }
    return num;
}

The GetItemText method uses ToString() on the object which returns null and so a message is sent with null parameter, which in turn returns an invalid pointer and you enter the second case which throws the exception.

Answer 2

When the underlying LB_ADDSTRING Windows API call fails, WinForms always returns an OutOfMemoryException. A comment in the .NET Framework Reference Source explains why:

// On some platforms (e.g. Win98), the ListBox control
// appears to return LB_ERR if there are a large number (>32000)
// of items. It doesn't appear to set error codes appropriately,
// so we'll have to assume that LB_ERR corresponds to item 
// overflow.
// 
throw new OutOfMemoryException(SR.GetString(SR.ListBoxItemOverflow));