this my code:
 
LineSymbolizer line = new LineSymbolizer();
line.SetWidth(10);
line.SetFillColor(Color.Blue);
line.SetOutline(Color.Red,1);
layer.Symbolizer = line;
 
produces blue line with BLACK border but not with red one.
and this code:
 
LineSymbolizer line = new LineSymbolizer(Color.Blue,Color.Red,10,DashStyle.Solid,LineCap.Flat);
layer.Symbolizer = line;
 
produces blue line with red border and with additional white border
so I can't create custom line in normal way only with tricks like CartographicStroke or others