Custom Web Part property : Drop down list

Create a user-defined enumeration method named CustomMode.

public enum CustomMode
{
Full,
Titles
}

Now create a custom property which will use our enumeration method:

private CustomMode displayMode= CustomMode.Full;
[Personalizable(PersonalizationScope.User), WebBrowsable(true), WebDisplayName("Display Mode"),
WebDescription("Which mode would u like to use to display the information?"),
Category("Custom Settings")]
public CustomMode DisplayMode{
get{return displayMode}
set{displayMode= value;}