public enum SomeType
    { 
        TYPE1,TYPE2, TYPE3
    }

    private void SetDropDownList()
    {
        System.Reflection.FieldInfo[] fis = typeof(SomeType).GetFields();
        foreach (System.Reflection.FieldInfo fi in fis)
        {
            if (fi.FieldType == typeof(SomeType))
            {
                DropDownList1.Items.Add(new ListItem(fi.Name));
            }
        }
    }