Make combobox act like drop-down list (C#)

Make combobox act like drop-down list (C#)

I'm using visual studio 2010 which supports .Net 4 and doesn't have
support for a Drop Down List. I am trying to get the same effect through a
combobox by disabling the ability for a user to enter free-form text, but
not sure how. How can I accomplish this goal?
ADDITIONAL INFO
Visual studio 2010 support goes to .NET 4
Drop down box support in forms was added in .NET 4.5
That's why I can't use a drop-down item nativity.
That's why I'm asking for help.