Question

I’m using LINQ’s Union method to combine two or more collections. After that I’m trying to apply sorting to the combined collection by calling OrderBy on a field that is common to the collections. Here is how I am applying sorting:

combinedCollection.OrderBy(row => row["common_field"]);

combinedCollection is defined as:

Enumerable<DataRow> combinedCollection;

I need the sorting to be applied to the entire combined collection. For some reason, that is not happening. Instead I see there is sorting applied on some other field separately within each ‘collection’ block within the combined collection

And idea why??

First Edit

foreach (....)
{
    if (combinedCollection != null)
    {
        combinedCollection = combinedCollection.Union(aCollection);
    }
    else
    {
        combinedCollection = aCollection;
    }

}

Second Edit

    _Cmd.CommandText = "SELECT     Person.Contact.FirstName, Person.Contact.LastName, Person.Address.City, DATEDIFF(YY, HumanResources.Employee.BirthDate, GETDATE()) AS Age"
    + " FROM         HumanResources.EmployeeAddress INNER JOIN"
    + " HumanResources.Employee ON HumanResources.EmployeeAddress.EmployeeID = HumanResources.Employee.EmployeeID INNER JOIN"
    + " Person.Address ON HumanResources.EmployeeAddress.AddressID = Person.Address.AddressID INNER JOIN"
    + " Person.Contact ON HumanResources.Employee.ContactID = Person.Contact.ContactID AND HumanResources.Employee.ContactID = Person.Contact.ContactID AND "
    + " HumanResources.Employee.ContactID = Person.Contact.ContactID AND HumanResources.Employee.ContactID = Person.Contact.ContactID";

    DataTable employeeTable = new DataTable();
    _Adpt.Fill(employeeTable);
    DataRow[] allRows = null;

    allRows = employeeTable.Select("");

    IEnumerable<DataRow> filteredEmployeeRows;

    filteredEmployeeRows = from row in allRows select row;

    // Declare a variable to hold the city-filtered rows and set it to null for now
    IEnumerable<DataRow> cityFilteredEmployeeRows = null;


    //Copy filtered rows into a data table
    DataTable filteredEmployeeTable = filteredEmployeeRows.CopyToDataTable<DataRow>();

    foreach (DataRowView city in CityListBox.SelectedItems)
    {
        // create an exact copy of the data table
        DataTable filteredEmployeeCopyTable = filteredEmployeeTable.Copy();

        // Enumerate it
        IEnumerable<DataRow> filteredEmployeeRowsInSingleCity = filteredEmployeeCopyTable.AsEnumerable();

        // Apply the city filter
        filteredEmployeeRowsInSingleCity = _ApplyCityFilter(filteredEmployeeRowsInSingleCity, city["City"].ToString());

        if (cityFilteredEmployeeRows != null)
        {
            // Combine the filtered rows for this city with the overall collection of rows
            cityFilteredEmployeeRows = cityFilteredEmployeeRows.Union(filteredEmployeeRowsInSingleCity);
        }
        else
        {
            cityFilteredEmployeeRows = filteredEmployeeRowsInSingleCity;
        }

    }

    //apply ordering
    cityFilteredEmployeeRows.OrderBy(row => row["Age"]);
    //cityFilteredEmployeeRows.OrderByDescending(row => row["Age"]);

    EmployeeGridView.DataSource = cityFilteredEmployeeRows.CopyToDataTable<DataRow>();

…….

private IEnumerable<DataRow> _ApplyCityFilter(IEnumerable<DataRow> filteredEmployeeRows, string city)
{
    IEnumerable<DataRow> temp = filteredEmployeeRows;

    filteredEmployeeRows = from row in temp
                           where row["City"].ToString() == city
                           select row;
    return filteredEmployeeRows;
}

Answer 1

I think you have a problem with the LINQ lazy evaluation, I would have to investigate to find out wich part causes the problem.

Using the foreach(var item...) in lazy functions has already bitten me (because when executed later they all reference the last iterated item), but in your case it doesn’t look like this is the problem.

To check it is the really the issue you can just use a DataRow[] in place of the IEnumerable<DataRow> and call .ToArray() after every LINQ function.

Edit: I’m not sure I got your code right but can’t you just use:

  var cities = CityListBox.SelectedItems.Cast<DataRowView>()
      .Select(city => city["City"].ToString())
      .ToArray();

  var rows = allRows
      .Where(r => cities.Contains((string)r["City"]))
      .OrderBy(r => (int?)r["Age"])
      .ToArray(); // if you want to evaluate directly, not mandatory