Question

How to write this LINQ expression in another .SelectMany() form ?

var result = 
                    from a in numbersA
                    where a < 3
                    from b in numbersB
                    where b < 5
                    select new { a, b };

?

var result = numbersA.Where(x => x < 3).Select.. ?

Answer 1

This is a rough translation of what the compiler would do:

var result = numbersA.Where(a => a < 3)
                     .SelectMany(a => numbersB, (a, b) => new { a, b })
                     .Where(z => z.b < 5)
                     .Select(z => new { z.a, z.b });

Now you can write this more efficiently as:

var result = numbersA.Where(a => a < 3)
                     .SelectMany(a => numbersB.Where(b => b < 5),
                                 (a, b) => new { a, b });

… but that’s not what the compiler would do. It’s not clear whether your aim is to see what the compiler does, or to just write a query.