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

Question

I have a IQueryable method as below:

public IQueryable<vmTest> TestMethod(long ID)
{
    return from m in db.table1
           join n in db.table2
           on m.table1_ID equals n.table1_ID into tabC
           from c in tabC
           join o in db.table3
           on m.table3_ID equals o.table3_ID
           where m.table1_ID.Equals(ID)
           select new vmTest{ field1 = m.xxx };
}

from the code above, m.xxx is a long type and in the vmTest there is a public long field1. There is error with m.xxx which saying Cannot implicitly convert type 'long?' to 'long'.An explicit conversion exists (are you missing a cast?).May I know what's wrong?

Additional information:
If i cast (long) in front of m.xxx, error will be dissapear but another problem occur when this query returning no value as it's wrong to cast long to null

Answer 1

You have several solutions:

• You can change vmTest class definition and replace field1 type from long to long?
• You can change your table in database and transforms xxx from NULL to NOT NULL then xxx will become a long

• You can use a condition in your query like

  new vmTest { field1 = m.xxx.HasValue ? m.xxx.Value : 0 }
  

More information on Nullables here

Answer 2

if you are sure of the types you want to use, you could try this:

public IQueryable<vmTest> TestMethod(long ID)
{
    return from m in db.table1
           join n in db.table2
           on m.table1_ID equals n.table1_ID into tabC
           from c in tabC
           join o in db.table3
           on m.table3_ID equals o.table3_ID
           where m.table1_ID.Equals(ID)
           select new vmTest
               { 
                    field1 = m.xxx.HasValue 
                        ? m.xxx.Value
                        : default(long) /*or something else, or an exception maybe... according to your expectation in this case*/ };
}