Question

I like to use linq to do a union on 3 tables.
Not sure why something like the following would not work:

var repdata = (from p in db.Table1
               select p)
                       .Union(from p in Table2
                        select p);

Answer 1

Union only works with the same element type. You could use:

var data = db.Table1.Select(p => new { p.Value1, p.Value2 })
     .Union(db.Table2.Select(q => new { q.Value1, q.Value2 })
     .Union(db.Table3.Select(r => new { Value1 = r.Alias1, Value2 = r.Other }); 

Here the anonymous type serves as a common type to project all three tables onto, so that Union can work.