Check this line in documentation under remarks This overload of the OleDbDataAdapter constructor uses the selectConnectionString parameter to set the SelectCommand property. However, it does not open the connection. You still must explicitly open the connection.
http://msdn.microsoft.com/en-us/library/2f8y4737(v=vs.110).aspx
I think that is your problem
Try this
string query =
@"SELECT EmpID, FirstName as [Name], LastName , EmployeePosition FROM Employees";
using (var odb = new OleDbConnection(connectionString))
{
odb.Open();
if (odb.State == ConnectionState.Open)
{
var dset = new DataTable();
var adapter = new OleDbDataAdapter();
var newCmd = new OleDbCommand(query, odb)
{
CommandType = CommandType.Text
};
adapter.SelectCommand = newCmd;
adapter.Fill(dset);
odb.Close();
}
}