I just found out the way to list out all the enum values. It's indeed very simple!
For example, I want to list out all the enum values of FileAccess.
string enumList = Enum.GetNames(typeof(FileAccess));
DropDownList1.DataSource = enumList;
I also found out how to convert the string representation value to the enum value too.
FileAccess fAccess = (FileAccess)Enum.Parse(typeof(FileAccess), DropDownList1.SelectedValue);
that's it. Isn't it simple?