I'm using NetTopologySuite to read a shapefile like below code:

        private FeatureDataTable GetFeatureTableFromShapefile(string shapeFilePath)
    {
        GeoAPI.GeometryServiceProvider.Instance = new NetTopologySuite.NtsGeometryServices();
        FeatureDataSet ds;
        using (var sf = new ShapeFile(shapeFilePath) {Encoding = Encoding.Default})
        {
            sf.Open();
            var ext = sf.GetExtents();
            ds = new FeatureDataSet();
            sf.ExecuteIntersectionQuery(ext, ds);
        }

        return ds.Tables[0];
    }

Now I want to merge all features and create only one feature from them.

Please note that I'm trying to create one feature and not something like multipolygon like below image:

For above image, i used this code, but it's not what i want, because there still a red line between to features that i tried to merge:

            var entities = new List<DbGeometry>(){ d1.Geom, d2.Geom};
        DbGeometry allBoundaries = null;
        for (var i = 0; i < entities.Count; i++)
        {
            if (i == 0) allBoundaries = entities[i].Boundary;
            else allBoundaries = allBoundaries.Union(entities[i].Boundary);
        }

So is there any way to accomplish this?