Hi Bassem,
The MSDN document describing the TextBox.AutoCompleteCustomSource property says:
The operating system might limit the number of custom strings that it can display at once.
So it seems that the auto complete custom source of the TextBox on your machine is limited to 300 strings. I suggest that you use another control, e.g. ComboBox or ListBox instead of the TextBox control on your form. Or you can add a button behind the TextBox. When the button is clicked, a form pops up showing all dictionary items in it.
Hope this helps.
If you have any question,please feel free to let me know.
Sincerely,
Linda Liu