How to obtain a Null value in combo boxes?

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To obtain a Null value in a combo box, you can follow these steps:

1. Add a new item to the combo box by typing in the word "Null" as the display text.
2. Set the value of this item to be null or an empty string.
3. Set the combo box's SelectedIndex property to -1, which will clear any selected item and display the Null item as the default value.

Alternatively, you can set the combo box's DataSource property to a list that includes a null value or an empty string as the first item. This way, users can select this item to indicate a null value.