IEnumerable OrderBy on a text field

__John_

Hi, When I try...

people.OrderBy(name => name.Name);

I get...

ex = {"An order by expression can only contain non-constant scalars that are order comparable by the server. The expression with type 'Text' is not order comparable."}

Can I get IEnumerable OrderBy to sort on a text field, using a predicate perhaps? Thanks.

Wayne Gaylard

There must be some mistake in your code prior to your orderby call, as you should be able to order by text. As a quick demo I did this

public partial class Form1 : Form
{
List<Person> sortedPeople;

    public Form1()
    {
        InitializeComponent();
        List<Person> people = new List<Person> {new Person("wayne"), new Person("sarah"), new Person("mark"), new Person("simon"), new Person("ashleigh"), new Person("dave"), new Person("connor"), new Person("bronwyn"), new Person("chantelle"), new Person("will"), new Person("chris")};
        sortedPeople = people.OrderBy(name = name.Name).ToList();
    }
}

class Person
{
    public Person(string name)
    {
        this.Name = name;
    }

    public string Name { get; set; }
}

and SortedPeople came out correctly

When I was a coder, we worked on algorithms. Today, we memorize APIs for countless libraries — those libraries have the algorithms - Eric Allman

manognya kota

Hi, try the below code snippet var newList = people.OrderBy(name => name.Name).ToList(); -Manognya __________________________________________________ $ God gives what is best.Not what all you wish :)

__John_

Hi, thanks for looking at this. Not sure what I am doing wrong. I tried...

         DataContext = new DataClasses1DataContext();

         var people = from name in DataContext.Table\_Peoples
                      where name.IQ < 3
                      select name;

         var peopleOrdered = people.OrderBy(name => name.Name).ToList();

'Name' is decared as follows...

[global::System.Data.Linq.Mapping.ColumnAttribute(Storage="_Name", DbType="Text", UpdateCheck=UpdateCheck.Never)]
public string Name
{
get
{
return this._Name;
}
set
{
if ((this._Name != value))
{
this._Name = value;
}
}
}

It is still thowing the same exception.

__John_

Ok I got it to work like this...

           DataContext = new DataClasses1DataContext();

           var people = from name in DataContext.Table\_Peoples
                        where name.IQ < 3
                        select name;

           List<Table\_People> listPeople = people.ToList();

           var peopleOrdered = listPeople.OrderBy(name => name.Name).ToList();

But I dont understand why I have to copy to a List first?

manognya kota

Quote:

__John_ wrote:

List<Table_People> listPeople = people.ToList(); var peopleOrdered = listPeople.OrderBy(name => name.Name).ToList();

To order the list, it requires the set of names to be in a format and we are storing it as the enumerable list.

-Manognya __________________________________________________ $ God gives what is best.Not what all you wish :)

__John_

Hi Manognya, I dont understand why List.OrderBy works when IEnumerable.OrderBy does not? I am not sure what you mean when you say 'in a format'? Thanks.

manognya kota

Hi John, The syntax of OrderBy is:

Public Shared Function OrderBy(Of TSource, TKey) ( _
source As IEnumerable(Of TSource), _
keySelector As Func(Of TSource, TKey) _
) As IOrderedEnumerable(Of TSource)

Parameters

source
Type: System.Collections.Generic.IEnumerable(Of TSource)
A sequence of values to order.

keySelector
Type: System.Func(Of TSource, TKey)
A function to extract a key from an element.

we need to convert the source to be of type

System.Collections.Generic.IEnumerable

Hope this helps.

-Manognya __________________________________________________ $ God gives what is best.Not what all you wish :)

manognya kota

Also, look at the link http://msdn.microsoft.com/en-us/library/bb534966.aspx#Y1297[^] Do not forget to vote the answer that serves the purpose ;)

-Manognya __________________________________________________ $ God gives what is best.Not what all you wish :)

__John_

Thanks, it is slowly becoming clear, I probably just need to think about it a bit more. I do sometimes find templates a bit confusing. +5

manognya kota

Agree..templates is always little tricky(for me either ;))...But, Once got..makes life easy:)

-Manognya __________________________________________________ $ God gives what is best.Not what all you wish :)

Vincent Blais

The difference is not the IEnumerable or the List but where the OrderBy take place. When you call ToList,or any greedy query operators, you execute your Ling query against your DB. After that, all Linq operation are executed in memory and Linq-to-object can do more things in than Linq-to-sql, or Linq-to-entities

Vince Remember the dead, fight for the living

__John_

Thanks Vince, that explains it a bit more. I think i can now understand what the debuger is showing me. It uses a 'lazy' stratergy, ie. only evaluating an expression or executing a function when the result is actually needed, am I right? BTW: How can I enumerate the results more that once? Hoping that is not too stupid a question :doh: Thanks - John.

__John_

http://msdn.microsoft.com/en-us/library/s793z9y2.aspx[^]

You cannot set Current to the first element of the collection again; you must create a new enumerator instance instead.

Seems a strange way of going about things but I am sure there is a reason :confused:

Vincent Blais

__John_ wrote:

It uses a 'lazy' strategy, ie. only evaluating an expression or executing a function when the result is actually needed, am I right?

Yes you are right. It's a principle of Linq to defer execution until is needed. And Linq also evaluate only the elements needed to return the result Let's take Wayne List and do some examples

List peoples = new List {new Person("wayne"), new Person("sarah"), new Person("mark"), new Person("simon"), new Person("ashleigh"), new Person("dave"), new Person("connor"), new Person("bronwyn"), new Person("chantelle"), new Person("will"), new Person("chris")};

int Count = people.Count(p => p.Name.StartsWith("w")); //<-- Count is a greedy operator and all persons are evaluated for a result of 2.

var firstperson = people.Where(p => p.Name.Length == 5).Take(3);
foreach (var p in firstperson) // <-- evaluation start where and only Wayne, Sarah, Mark and Simon will be evaluated. The rest of the list is left alone
{
Console.WriteLine(p.Name);
}

// And to show you when Linq expression are evaluated, try
var firstperson2 = people.Where(p => p.Name.Length == 5).Take(3);
people.Insert(1, new Person("Vince"));

foreach (var p in firstperson) // <-- evaluation start where and only Wayne, Vince ans Sarah, will be evaluated. The rest of the list is left alone
{
Console.WriteLine(p.Name);
}

http://blogs.msdn.com/b/charlie/archive/2007/12/09/deferred-execution.aspx[^]

__John_ wrote:

BTW: How can I enumerate the results more that once?

If you use a Enumerator, you can use Reset to set the enumerator to its initial position, which is before the first element in the collection. But if you use a foreach loop, you can reuse an Enumerable many times. The foreach loop will start at the beginning every time.

Vince Remember the dead, fight for the living

__John_

+5 Thanks Vince.

Wayne Gaylard

Nice examples - thanks!

When I was a coder, we worked on algorithms. Today, we memorize APIs for countless libraries — those libraries have the algorithms - Eric Allman

manognya kota

Nice examples.Thanks.Its more clear now.

-Manognya __________________________________________________ $ God gives what is best.Not what all you wish :)