Can a foreach iteration variable not change for ANY reason?

I have a foreach statement with an iteration variable named "rcnesw".

foreach( RCNESW rcnesw in shiftersave )
{
    TestG.ShiftQuad( rcQuads, rcnesw.row, rcnesw.col, GO_.Up);

    // other code...
}

During debugging the values of rcnesw.row and rcnesw.col CHANGE as a result of the call to ShiftQuad -- I thought the foreach loop variable was readonly. Is it ever not readonly?

Note: the TestG.ShiftQuad method also has a foreach loop in it with its own local (I thought local) variable which has the very same name and type: RCNESW rcnesw. That value seems to transmit back up to the calling foreach location.

(=====edit=====adding=====)

First, thanks to all for the info on the "mutable" nature of the "readonly" variable. That clarifies the situation (that the C#/.NET docs do not).

Further info on the ShiftQuad method, it is:

public void ShiftQuad( List<RCPair> rcList, int row, int col, int dir )

And inside that method nothing changes row or col. ShiftQuad declares its own rcnesw:

RCNESW rcnesw = null;

Or does it? Does that statement not create a new instance of RCNESW? Because when that method makes rcnesw.row and rcnesw.col change then the calling method receives those changes apparently.

public class RCNESW
{
    public int row;
    public int col;
    public bool bNorth;
    public bool bEast;
    public bool bSouth;
    public bool bWest;
}

At this point I still think it is a compiler bug. Changes to a local variable should not emerge at the caller through variables that are called by value.

(====edit====solved===) This issue was really related not to local vs non-local (or pass by value), the screwup was in how some lists became aliases for the same objects.