How to extract polygons in shp

[JeanCollas]

I have tried many times in different ways, I have not been able to extract valid polygons shp from for example regions of a country.

Some of what I have tried so far :

public void Extract(string fileNameIn, string folderOut)
{
	using (var fileStream = new FileInfo(fileNameIn).OpenRead())
	{
		var source = new PBFOsmStreamSource(fileStream);

		var f = (from s in source
				 where s.Tags.Contains("boundary", "administrative")
				 where s.Tags.Contains("admin_level", "3")
				 where s.Tags.ContainsKey("name")
				 where !s.Tags.ContainsKey("highway")
				 select s)
                                // .Take(1)
				 .ToFeatureSource();

		var fileOut = Path.Combine(folderOut, f.ToString());

		// build feature collection.
		var featureCollection = new FeatureCollection();
		var attributesTable = new AttributesTable();

		var geoCount = 0;
		foreach (var feature in f)
		{ // make sure there is a constant # of attributes with the same names before writing the shapefile.
			featureCollection.Add(new Feature(feature.Geometry, attributesTable));
			geoCount++;
		}
geoCount.Dump();
		if (geoCount > 0)
		{


			// convert to shape.
			var header = NetTopologySuite.IO.ShapefileDataWriter.GetHeader(featureCollection.First(), featureCollection.Count);
			var shapeWriter = new ShapefileDataWriter(fileOut + ".shp", new GeometryFactory())
			{
				Header = header
			};
			shapeWriter.Write(featureCollection);
		}
	}
}

This takes ~15min applied to an extract of France, but no relevant result.

Any recommendation ?