Its been working fine for ages, I spend a couple of days on unrelated parts
of the system, and I can no longer write files (or read old ones).

The error message I get is "There was an error reflecting type
WindowsApplication1.Form1.savestructure'.".

Here is the code, which has worked fine for ages:

      public class savestructure
       {
           public List<textaslot> alist;
           public int speed, size;
           public int squareorcircle;
       }

(textaslot is an awful class, with lots of public members. However, I
haven't added very much into it in the last few days, and taking what I have
doesn't seem to help, although it was a lot of work to try. Nor can I
understand why this would fail on write, so its not a compatibility issue.)

.
.

     private void savethisconfig()
       {
           savestructure thissave = new savestructure();
           thissave.alist = slotcollection;
           thissave.speed = (int)speedUpDown.Value;
           thissave.squareorcircle = mainrotatortype;
           thissave.size = (int)sizeUpDown.Value;
           serialiseandsave(thissave);
       }

public void serialiseandsave(savestructure allslots)
       {

           XmlSerializer mySerializer = new
XmlSerializer(typeof(savestructure));

... and that's where it crashes.

This is really spooking me, as it also means I can't load any of the dozens
of files I have created. I have no idea where to go after getting this
error - what to try.

Any suggestions?