why do my eating onions rot when i lift them

have you hung them some were were they can dry, like a shed or garage?

If you have lifted them and pulled them out rather than used a fork to gentle lift them out, this can damage the root plant and let disease in,

I had a good crop of Ailsa Craig this year. You may have grown yours in the same place as last year. There is no sense in hanging diseased onions in a garage or shed. They may have smut which stays in the ground for years. Burn badly affected onions. Make chutney with what you still have.

When did you actually lift them Greenhouse? We lifted ours in lat August and laid out to dry in the greenhouse for a couple of weeks before putting in a net sack and hanging. As Dr B has already said if they are rotting bin them using any bits you can but do it NOW the situation will only get worse. You can freeze the healthy bits diced or sliced for cooking with later. Don't be tempted to grow your onions in the same place next year.

the reason I said what I did is because it is normal practis,some peopel put them in a box were they rot,I feal it best to find out the way they have been treated befor we pass judgment on them that is why I asked' did you miss the question mark, by the way ailsa craig are a grate keeping onion I have grown them for fifty odd years , like your choise

Smut is just one disease you can miss when lifting your crop. It could be neck rot or white rot, all when conditions are wet. Ailsa Craig is one of the resistant varieties.

You're not alone Greenhouse, I too have had big problems with my onions. On looking for causes on line I think that it's something called white rot. I can't be certain but the symptoms are there. see here.
http://www.which.co.uk/documents/pdf/onion-white-rot-153658.pdf