A .prj file is loaded automatically. If the AUTHORITY tag is missed, the Code property will return 0, but this does not mean that the .prj file has been loaded unsuccessfully (only if Layer.CoordinateSystem is null).
To transform coordinates on-the-fly, set coordinate systems to Map.CoordinateSystem and Layer.CoordinateSystem(unless the layer has a .prj file). This way coordinates will be transformed implicitly. All other coordinates must be transformed to map/layer coordinate systems explicitly by using the methods of the CoordSystem class.
Instead of the .prj file, you may use the pre-defined constants:
CoordSystem sys = new CoordSystem(CoordSystemCode.PCS_NAD1983StatePlaneMarylandFIPS1900Feet);