C# Linq-SQL query result returning nothing

Question

I'm fairly new to LINQ (and SQL at that). When trying to query my SQL Database in C# "Supervisors" (which contains only one column "Names" made of nvarchar(50) variables, none of which are Null), supvName ends up being an empty list. If I don't cast it as a List, supvName is of typeSystem.Data.EnumerableRowCollection<string> if that helps.

public addNewEmp()
{
    InitializeComponent();
    using (TestDataSet db = new TestDataSet())
    {
        var supvName = (from n in db.Supervisors
                        select n.Names).ToList();
        foreach (string n in supvName)
        {
            supv_cbox.Items.Add(n);
        }
    }
}

Even when using a Where statement, that one result doesn't show up, so I'm sure it's something simple in my code that I just can't seem to figure out. I've already tried using AsEnumerable() which didn't change anything.

EDIT: I'm doing this in VS 2010 WPF. Also when I Preview Data in TestDataSet.xsd, it does return the all the data in the Database.

Solution: The problem was when I used a DataSet. When I used a DataContext instead it worked perfectly fine. Thanks to your DataSet or DataContext question lazyberezovsky or I never would have tried that. Using the following works:

var supvName = db.Supervisors.Select(m => m.Names);
supv_cbox.ItemsSource = supvName;

Thanks Surjah Singh too.

TestDataSet is a DataSet or DataContext?

Sergey Berezovskiy
– Sergey Berezovskiy

2013-09-17 14:08:30 +00:00
Commented Sep 17, 2013 at 14:08 — Sergey Berezovskiy
– Sergey Berezovskiy, Commented Sep 17, 2013 at 14:08
TestDataSet is a DataSet

PRX
– PRX

2013-09-17 14:26:16 +00:00
Commented Sep 17, 2013 at 14:26 — PRX
– PRX, Commented Sep 17, 2013 at 14:26
Then my solution below should work for you

Sergey Berezovskiy
– Sergey Berezovskiy

2013-09-17 14:28:31 +00:00
Commented Sep 17, 2013 at 14:28 — Sergey Berezovskiy
– Sergey Berezovskiy, Commented Sep 17, 2013 at 14:28

Sergey Berezovskiy · Accepted Answer · 2013-09-17 14:16:07Z

1

When you are enumerating over DataTable with Linq to DataSet, you should call AsEnumerable() on datatable and use Field<T> extension to get column value:

 var supvName = (from r in db.Supervisors.AsEnumerable()
                 select r.Field<string>("Names")).ToList();

BTW query variable r will be of DataRow type.

Your code can be simplified to:

 var names = db.Supervisors.AsEnumerable().Select(r => r.Field<string>("Names"));
 supv_cbox.DataSource = names.ToList();

edited Sep 17, 2013 at 14:16

answered Sep 17, 2013 at 14:05

Sergey Berezovskiy

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2 Comments

PRX Over a year ago

Unfortunately supvName is still Count = 0 (tried both ways of implementing it). Could the problem possible be that I'm doing this in WPF instead of WinForm (I substituted DataSource out for DataContext and ItemsSource since i wasn't sure which)?

PRX Over a year ago

Recreated the window in WinForm and same issue XD Going to try using SQL commands and then DataContext to see if that works

Suraj Singh · Accepted Answer · 2013-09-17 15:06:15Z

0

    //Var supvName = Supervisors.Select(m=>m.Names);

   var supvName = from s in Supervisors.Tables[0].AsEnumerable()
                     select s.Field<string>("Names");

edited Sep 17, 2013 at 15:06

answered Sep 17, 2013 at 14:32

Suraj Singh

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