AutoCAD .NET :: How To Get ObjectIdCollection Of All Entities In A Certain Layer

How could I get the ObjectIdCollection of all entities (points, polyline) in a certain layer?

I am using the following code at this moment.
 
private ObjectIdCollection GetEntitiesOnLayer(string layerName)
{
Editor ed = doc.Editor;
TypedValue[] tvs = new TypedValue[1] { new TypedValue((int)DxfCode.LayerName, layerName) };
SelectionFilter sf = new SelectionFilter(tvs);
PromptSelectionResult psr = ed.SelectAll(sf);

[code]....

But it seems the function fetches more ObjectId than I expected.For example, if I draw 3 points and 2 polylines in "A" layer.

ObjectIdCollection obc = GetEntitiesOnLayer("A");

int i = obc.Count;

I was expecting i to be 5 (3 points and 2 polylines), but i seems to be more than 5.