spgridview

[C# Programming]

i have spgridview thats allowgroupfield is true,
i write this:
gridView.AllowGrouping = true;
gridView.GroupField = "YearMonth";
gridView.GroupFieldDisplayName = "تار?خ ثبت";
gridView.AllowGroupCollapse = true;

i want when my form run spgridview show expand not colapse but i dont know what should i do, plz help