If Else in LINQ

This might work...

from p in db.products
    select new
    {
        Owner = (p.price > 0 ?
            from q in db.Users select q.Name :
            from r in db.ExternalUsers select r.Name)
    }

I assume from db that this is LINQ-to-SQL / Entity Framework / similar (not LINQ-to-Objects);

Generally, you do better with the conditional syntax ( a ? b : c) - however, I don't know if it will work with your different queries like that (after all, how would your write the TSQL?).

For a trivial example of the type of thing you can do:

select new {p.PriceID, Type = p.Price > 0 ? "debit" : "credit" };

You can do much richer things, but I really doubt you can pick the table in the conditional. You're welcome to try, of course...

Answer above is not suitable for complicate Linq expression. All you need is:

// set up the "main query"
var test = from p in _db.test select _db.test;
// if str1 is not null, add a where-condition
if(str1 != null)
{
    test = test.Where(p => p.test == str);
}

 var result = _context.Employees
                .Where(x => !x.IsDeleted)
                .Where(x => x.ClientId > (clientId > 0 ? clientId - 1 : -1))
                .Where(x => x.ClientId < (clientId > 0 ? clientId + 1 : 1000))
                .Where(x => x.ContractorFlag == employeeFlag);
            return result;

If clientId = 0 we want ALL employees,. but for any clientId between 1 and 999 we want only clients with that ID. I was having issues with seperate LINQ statements not being the same (Deleted/Clients filters need to be on all queries), so by add these two lines it works (all be it until we have 999+ clients - which would be a happy re-factor day!!