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

Question

First, I found a solution for the exception. I am more curious why this caused a specific exception.

In my script, I add POCO to the ListBox like this:

myListBox.Items.Add(myPOCO); 

This OutOfMemoryException . The problem was that the ToString from POCO was returning null . I added a string.IsNullOrEmpty check to return a "safe" value when null and the exception passed.

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

EDIT : elements are added to the for loop.

Full call stack (company links removed). One note - the list box is empty when it 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 due to the way the 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() for an object that returns null , and therefore the message is sent with the null parameter, which in turn returns an invalid pointer, and you introduce the second case that throws an exception.

Answer 2

When a basic LB_ADDSTRING Windows API call fails, WinForms always returns an OutOfMemoryException . A comment in the source source of the .NET Framework explains why:

 // On some platforms (eg 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));