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.ObjectColle…

Accepted Answer

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.

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

Solution

Problem