Thank you so much for your help, Nagma. I have tried your code, but doesn't work to me. I always got error messages. Please see the image. My code is like the follows. The new data can be saved to the offline database after the following code runs. But when I synchronized the offline geodatabse, the new data won't be synchronized to our geodatabase, but edited data can.
Private _feature As ArcGISFeature = Nothing
_feature = Offlinedatabase.GeodatabaseFeatureTables(0).CreateFeature()
_feature.Geometry = ptloc
_feature.SetAttributeValue(Constants.COL_CREATED_BY, Environment.UserName)
_feature.SetAttributeValue(Constants.COL_CREATION_DATE, Convert.ToDateTime(DateTime.Now.ToString(Constants.formatDateTime)))
Await _feature.FeatureTable.AddFeatureAsync(_feature)