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A combination box, or combo box, is a specialized control coded in modern object-oriented languages such as Java, .Net and Microsoft Visual Basic. It is a drop-down list box that includes an opportunity for the user to either choose an option from the list, or type in their own option in the top line of the text box.
Combination boxes are also known as drop-down combination boxes.
The combination box allows for a flexible kind of input. Here, users can either make a predetermined choice, or input their own information into a form.
As a result, it requires complex programming to make use of that input. Programmers and developers have to treat the user-submitted input differently than the drop-down list input. For example, a user in a form might choose from three predetermined elements: red, blue or yellow. Or, they might enter something like "purple." They also might type in something like "cat," "dog" or "duck." They might misspell the word purple, or they might include special characters. This is the challenge of interpreting input from a combination box control.
Some IT pros may use the terms "combination box" and "drop-down list box" interchangeably. Here, it might be helpful to utilize a term like "non-editable drop-down box" to describe a purely drop-down list box that is not a combination box and does not have that user-submitted input functionality.