public void Reshape( 
   SelectionSet features,
   Geometry reshapeGeometry
)

var reshapeFeatures = new EditOperation();
reshapeFeatures.Name = "Reshape Features";

reshapeFeatures.Reshape(featureLayer, oid, modifyLine);

//Reshape a set of features that intersect some geometry....

//at 2.x - var selFeatures = MapView.Active.GetFeatures(modifyLine).Select(
//    k => new KeyValuePair<MapMember, List<long>>(k.Key as MapMember, k.Value));
//reshapeFeatures.Reshape(selFeatures, modifyLine);

reshapeFeatures.Reshape(MapView.Active.GetFeatures(modifyLine), modifyLine);

//Execute to execute the operation
//Must be called within QueuedTask.Run
if (!reshapeFeatures.IsEmpty)
{
  var result = reshapeFeatures.Execute(); //Execute and ExecuteAsync will return true if the operation was successful and false if not
}

//or use async flavor
//await reshapeFeatures.ExecuteAsync();