When you create a Combo Box in Visual Studio  , the default behaviour is that it will show the list and it will also allow the data entry. So a user can type something in instead of selecting the options listed in the Combo Box. This can cause issues as you might want the user to only select the valid options which are listed in your combo box.

Here is a simple way to achieve this behavior. Highlight the Combo Box on your form. The properties window will be show.

Change the Drop Down Style to  DropDownList as shown below.

Your combo box will not allow the users to type in any free text. they will have to select the options listed in your combo box.