How to make an anonymous type property nullable in the select of a linq to sql query

Question

Consider this simplified example query:

dt = (
  from O in ...
  ...
   select new
   {
      O.A,
      B = 0
   }).ToList().ToDataTable()

Where .ToDataTable() comes from Convert generic List/Enumerable to DataTable?

Assuming that table O does not have an attribute called B but I want to have a column called B whose values I will fill in later. This works fine when B get number values sent to it but i actually want to send it int? values and at present it crashes if it gets send a null.

Is there a way to select a new column (e.g. B) such that it is a nullable type?

You just set `B = 0`, how does it crash? I don't understand. — Sriram Sakthivel, May 14 '15 at 11:17
@Bjørn-RogerKringsjå thanks, I got it working this way. Had to do some casting to `object` later on though because the `DataTable` I put this in wants `DBNull.Value` instead of `null`. But it works. Thanks! — Dan, May 14 '15 at 11:17
@SriramSakthivel it crashes later on when I manually try asign `null` to column `B` (which is in a `DataTable` by then) — Dan, May 14 '15 at 11:18
You can't assign anonymous type's properties once it is created. It is immutable. How does the `DataTable` comes to picture here? Your question is unclear really. — Sriram Sakthivel, May 14 '15 at 11:23

score 2 · Answer 1 · answered May 14 '15 at 11:20

2

Try:

select new
{
    O.A,
    B = (int?)0
}

answered May 14 '15 at 11:20

Martin Mulder

12,642
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score 2 · Accepted Answer · answered May 14 '15 at 11:21

2

You can do like this,

var testNull = (from student in studentList
                select new
                {
                    RollNo = default(int?),
                    Name = student.Name
                }).ToList();

But better we can create concrete type instead of depending upon a anonymous type based on value.:-)

answered May 14 '15 at 11:21

Sivaprasath

400
1
7

What do you mean by that last sentence please? – Dan May 14 '15 at 11:38

score 1 · Answer 3 · answered May 14 '15 at 11:18

1

Since the type of anonymous type members is inferred make it infer the right thing: B = (int?)null.

Not sure what you mean by "fill in later". If you want B to be mutable you need to create a class.

answered May 14 '15 at 11:18

usr

168,620
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Apologies, by "fill it in later" I mean I convert `q` to a DataTable and then manually assign values into column `B` of the `DataTable` – Dan May 14 '15 at 11:22

How to make an anonymous type property nullable in the select of a linq to sql query

3 Answers3