Got the solution! as shared project is for both windows app and windows phone. Previously I thought that ContactStore is for both but checking the msdn it appeared that it is available only for windows phone 8.1 only. So I have apply Windows phone directive to in my shared project. here is how
#if WINDOWS_PHONE_APP
ContactStore contactStore = await ContactManager.RequestStoreAsync();
IReadOnlyList<Contact> contacts = null;
// Find all contacts
contacts = await contactStore.FindContactsAsync();
foreach (var item in contacts)
{
if (!string.IsNullOrEmpty(item.FirstName) && !string.IsNullOrEmpty(item.LastName))
{
var acontact = new Contact() { Name = item.FirstName + " " + item.MiddleName + " " + item.LastName, };
if (item.Thumbnail != null)
{
var thumnailStream = await item.Thumbnail.OpenReadAsync();
BitmapImage thumbImage = new BitmapImage();
thumbImage.SetSource(thumnailStream);
}
myContactsList.Add(acontact);
}
}
#else
//do windows phone logic here
#endif