By using this GeometryBag you can selected multiple object within another geometry object. this method give you fast result.

public IGeometryBag Create_GeometryBag_from_FeatureClass(IFeatureClass pFeatureClass)
{
IGeometryBag pGeometryBag = new GeometryBagClass();
IGeoDataset pGeoDataset = (IGeoDataset)pFeatureClass;
ISpatialReference pSpatialReference = pGeoDataset.SpatialReference;
pGeometryBag.SpatialReference = pSpatialReference;
IGeometryCollection pGeometryCollection = (IGeometryCollection)pGeometryBag;
object MissingType = Type.Missing;
IFeatureCursor pFeatureCursor = pFeatureClass.Search(null, false);
IFeature pFeature = pFeatureCursor.NextFeature();
while (pFeature != null)
{
pGeometryCollection.AddGeometry(pFeature.Shape, ref MissingType, ref MissingType);
pFeature = pFeatureCursor.NextFeature();
}
Marshal.ReleaseComObject(pFeatureCursor);
return pGeometryBag;
}