public int PointCount {get;}

// Use a builderEx convenience method or use a builderEx constructor.

List<MapPoint> list = new List<MapPoint>();
list.Add(MapPointBuilderEx.CreateMapPoint(1.0, 1.0));
list.Add(MapPointBuilderEx.CreateMapPoint(1.0, 2.0));
list.Add(MapPointBuilderEx.CreateMapPoint(2.0, 2.0));
list.Add(MapPointBuilderEx.CreateMapPoint(2.0, 1.0));

// use the builderEx constructors which don't need to run on the MCT.
// use AttributeFlags.NoAttributes since we have 2d points in the list
MultipointBuilderEx builderEx = new MultipointBuilderEx(list, AttributeFlags.None);
Multipoint multiPoint = builderEx.ToGeometry() as Multipoint;
int ptCount = builderEx.PointCount;

// builderEx convenience methods don't need to run on the MCT
multiPoint = MultipointBuilderEx.CreateMultipoint(list);
// multiPoint.HasZ, HasM, HasID will be false - the attributes are determined 
//    based on the attribute state of the points in the list

// or specifically set the state
multiPoint = MultipointBuilderEx.CreateMultipoint(list, AttributeFlags.None);
// multiPoint.HasM = false

multiPoint = MultipointBuilderEx.CreateMultipoint(list, AttributeFlags.HasM);
// multiPoint.HasM = true

ptCount = multiPoint.PointCount;