1 to 1 mapping may be null

[xclayl]

We referencing a back reference in a 1 to 1 association, the back reference record may not exist, so the join should be a left join, not an inner join. For example:

from p in cxt.Products
select new {
   p.ShoeDetail
}

Not all products have a ShoeDetail record (only shoes have a ShoeDetail record)

[igor-tkachev]

Use CanBeNull field of the Association attribute.