With regards to the code by miki on Jun 7 2019

Document doc = Application.DocumentManager.MdiActiveDocument;
Editor ed = doc.Editor;

int polylineState = 1; // 0 - closed, 1 - open
TypedValue[] vals= new TypedValue[]
{
    new TypedValue((int)DxfCode.Operator, "<or"),

    // This catches Polyline object.
    new TypedValue((int)DxfCode.Operator, "<and"),
    new TypedValue((int)DxfCode.Start, "LWPOLYLINE"),
    new TypedValue(70, polylineState),
    new TypedValue((int)DxfCode.Operator, "and>"),

    new TypedValue((int)DxfCode.Operator, "<and"),
    new TypedValue((int)DxfCode.Start, "POLYLINE"),
    new TypedValue((int)DxfCode.Operator, "<or"),
    // This catches Polyline2d object.
    new TypedValue(70, polylineState),
    // This catches Polyline3d object.
    new TypedValue(70, 8|polylineState),
    new TypedValue((int)DxfCode.Operator, "or>"),
    new TypedValue((int)DxfCode.Operator, "and>"),

    new TypedValue((int)DxfCode.Operator, "or>"),
};
SelectionFilter filter = new SelectionFilter(vals);
PromptSelectionResult prompt = ed.GetSelection(filter);

// If the prompt status is OK, objects were selected
if (prompt.Status == PromptStatus.OK)
{
    SelectionSet sset = prompt.Value;
    Application.ShowAlertDialog($"Number of objects selected: {sset.Count.ToString()}");
}
else
{
    Application.ShowAlertDialog("Number of objects selected: 0");
}

I have tested and made corrections on the POLYLINE DXF CODE for Closed Polyline or Open Polyline

In LISP

Closed Polygon

(setq var (ssget "X" '((-4 . "<and")(-4 . "<or")(-4 . "<and")(0 . "POLYLINE")(-4 . "&")(70 . 1)(-4 . "and>")(-4 . "<and")(0 . "LWPOLYLINE")(-4 . "&")(70 . 1)(-4 . "and>")(-4 . "or>")(-4 . "<or")(410 . "MODEL")(67 . 0)(67 . 1)(-4 . "or>")(8 . "LAYERNAME")(-4 . "and>"))))

Open Polygon

(setq var (ssget "X" '((-4 . "<and")(-4 . "<or")(-4 . "<and")(0 . "POLYLINE")(-4 . "&")(70 . 0)(-4 . "and>")(-4 . "<and")(0 . "LWPOLYLINE")(-4 . "&")(70 . 0)(-4 . "and>")(-4 . "or>")(-4 . "<or")(410 . "MODEL")(67 . 0)(67 . 1)(-4 . "or>")(8 . "V-QURY-BNDY-LEGN")(-4 . "and>"))))

In C#

                typedValue = new List<TypedValue>{

                    new TypedValue(Convert.ToInt32(DxfCode.Operator), "<and"),
                    new TypedValue(Convert.ToInt32(DxfCode.Operator), "<or"),
                    new TypedValue(Convert.ToInt32(DxfCode.Operator), "<and"),
                    new TypedValue(Convert.ToInt32(DxfCode.Start), "POLYLINE"),
                    new TypedValue(Convert.ToInt32(DxfCode.Operator), "&"),
                    new TypedValue(Convert.ToInt32(DxfCode.Int16), 1),
                    new TypedValue(Convert.ToInt32(DxfCode.Operator), "and>"),
                    new TypedValue(Convert.ToInt32(DxfCode.Operator), "<and"),
                    new TypedValue(Convert.ToInt32(DxfCode.Start), "LWPOLYLINE"),
                    new TypedValue(Convert.ToInt32(DxfCode.Operator), "&"),
                    new TypedValue(Convert.ToInt32(DxfCode.Int16), 1),
                    new TypedValue(Convert.ToInt32(DxfCode.Operator), "and>"),
                    new TypedValue(Convert.ToInt32(DxfCode.Operator), "or>"),
                    new TypedValue(Convert.ToInt32(DxfCode.Operator), "<or"),
                    new TypedValue(Convert.ToInt32(DxfCode.LayoutName), "MODEL"),
                    new TypedValue(Convert.ToInt32(DxfCode.ViewportVisibility), 0),
                    new TypedValue(Convert.ToInt32(DxfCode.ViewportVisibility), 1),
                    new TypedValue(Convert.ToInt32(DxfCode.Operator), "or>"),
                    new TypedValue(Convert.ToInt32(DxfCode.LayerName), "LAYERNAME"),
                    new TypedValue(Convert.ToInt32(DxfCode.Operator), "and>")
                };