c# - Issue related to IQueryable method (Cannot implicitly cast type 'long?' to 'long' -

i have iqueryable method below:

public iqueryable<vmtest> testmethod(long id) {     return m in db.table1            join n in db.table2            on m.table1_id equals n.table1_id tabc            c in tabc            join o in db.table3            on m.table3_id equals o.table3_id            m.table1_id.equals(id)            select new vmtest{ field1 = m.xxx }; } 

from code above, m.xxx long type , in vmtest there public long field1. there error m.xxx saying cannot implicitly convert type 'long?' 'long'.an explicit conversion exists (are missing cast?).may know what's wrong?

additional information:
if cast (long) in front of m.xxx, error dissapear problem occur when query returning no value it's wrong cast long null

you have several solutions:

• you can change vmtest class definition , replace field1 type long long?
• you can change table in database , transforms xxx null not null xxx become long

• you can use condition in query like

  new vmtest { field1 = m.xxx.hasvalue ? m.xxx.value : 0 } 

more information on nullables here