Filling a DataSet or a DataTable from a LINQ query result set

Question

How do you expose a LINQ query as an ASMX web service?
Usually, from the business tier, I can return a typed DataSet or a DataTable which can be serialized for transport over ASMX.

How can I do the same for a LINQ query?
Is there a way to populate a typed DataSet or a DataTable via a LINQ query?

public static MyDataTable CallMySproc()
{
    string conn = "...";

    MyDatabaseDataContext db = new MyDatabaseDataContext(conn);
    MyDataTable dt = new MyDataTable();

    // execute a sproc via LINQ
    var query = from dr
                in db.MySproc().AsEnumerable
                select dr;

    // copy LINQ query resultset into a DataTable -this does not work !    
    dt = query.CopyToDataTable();
   
    return dt;
}

How could I put the result set of a LINQ query into a DataSet or a DataTable?
Alternatively, can the LINQ query be serializable so that I can expose it as an ASMX web service?

Caoutchouc

Answer 1

As mentioned in the question, IEnumerable has a CopyToDataTable method:

IEnumerable<DataRow> query =
    from order in orders.AsEnumerable()
    where order.Field<DateTime>("OrderDate") > new DateTime(2001, 8, 1)
    select order;

// Create a table from the query.
DataTable boundTable = query.CopyToDataTable<DataRow>();

Why won't that work for you?

Tinkling

To everyone wondering why CopyToDataTable() doesn't work on their machine: This function is not part of .NET 3.5 SP1 nor will it be of .NET 4.0; it has been restricted to IEnumerable<DataRows> and does not work for IEnumerable<T> -- bit.ly/dL0G5 – Hanshaw

Answer 2

To perform this query against a DataContext class, you'll need to do the following:

MyDataContext db = new MyDataContext();
IEnumerable<DataRow> query = 
    (from order in db.Orders.AsEnumerable()
        select new
        {
            order.Property,
            order.Property2
        })
    as IEnumerable<DataRow>;
return query.CopyToDataTable<DataRow>();

Without the as IEnumerable<DataRow>; you will see the following compilation error:

Cannot implicitly convert type 'System.Collections.Generic.IEnumerable' to 'System.Collections.Generic.IEnumerable'. An explicit conversion exists (are you missing a cast?)

Losing

Answer 3

Make a set of Data Transfer Objects, a couple of mappers, and return that via the .asmx.
You should never expose the database objects directly, as a change in the procedure schema will propagate to the web service consumer without you noticing it.

Insusceptible

Answer 4

If you use a return type of IEnumerable, you can return your query variable directly.

Upperclassman

Answer 5

Create a class object and return a list(T) of the query.

Sauerbraten

Answer 6

If you use IEnumerable as the return type, it will return your query variable directly.

MyDataContext db = new MyDataContext();
IEnumerable<DataRow> query = 
    (from order in db.Orders.AsEnumerable()
        select new
        {
            order.Property,
            order.Property2
        })
    as IEnumerable<DataRow>;
return query.CopyToDataTable<DataRow>();

Hegarty

Answer 7

For the sake of completeness, these solutions do not work for EF Core (at least not for EF Core 2.2). Casting to IEnumerable<DataRow>, as suggested in the other answers here, fails. Implementing this class and extension methods worked for me https://learn.microsoft.com/en-us/dotnet/framework/data/adonet/implement-copytodatatable-where-type-not-a-datarow.

Why it's not built into EF Core, I have not idea.

Judson