Filling a DataSet or 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 DataTable which can be seri

Answer 1

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

Answer 2

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

Answer 3

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://docs.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.

Answer 4

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?

Answer 5

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>();

Answer 6

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.