Strange, IEnumerable.ToList() creates entirely new objects

Question

I know IEnumerable.ToList() is supposed to create a new List, but with the items pointing to the same original items in the IEnumerable, as discussed at ToList()-- Does it Create a New List?

However, I'm getting some strange behavior with my code using VS 2012; WPF; and .NET 4.0. It started when IEnumerable.SequenceEquals() seemed not to work as I expected. I dug around with my QuickWatch dialog, and, unbelievably, the following statement evaluates to false:

this.Items.First () == this.Items.ToList ()[ 0 ]

I even tried:

this.Items.ToList ().IndexOf(this.Items.First ())

which evaluated to -1.

Items is declared as a property on a WPF custom control, like so:

public static readonly DependencyProperty ItemsProperty = DependencyProperty.Register (
        "Items", 
        typeof ( IEnumerable<UserLayoutType> ), 
        typeof ( UserLayoutSelectorControl ),
        new FrameworkPropertyMetadata ( null, FrameworkPropertyMetadataOptions.AffectsRender, UserLayoutSelectorControl.PropertyChanged ) );


public IEnumerable<UserLayoutType> Items
{
    get
    {
        return ( IEnumerable<UserLayoutType> ) this.GetValue ( UserLayoutSelectorControl.ItemsProperty );
    }
    set
    {    
        this.SetValue ( UserLayoutSelectorControl.ItemsProperty, value );                
    }
}

UserLayoutType is simply a class generated by the XSD tool, with the following declaration:

// 
// This source code was auto-generated by xsd, Version=4.0.30319.17929.
// 
namespace MyAssays.UserLayoutCore.UserLayoutUtility {
    using System.Xml.Serialization;


    /// <remarks/>
    [System.CodeDom.Compiler.GeneratedCodeAttribute("xsd", "4.0.30319.17929")]
    [System.SerializableAttribute()]
    [System.Diagnostics.DebuggerStepThroughAttribute()]
    [System.ComponentModel.DesignerCategoryAttribute("code")]
    [System.Xml.Serialization.XmlRootAttribute("UserLayout", Namespace="", IsNullable=false)]
    public partial class UserLayoutType {

This are the methods on a factory class that create the UserLayoutType Items in the first place:

public static IEnumerable<UserLayoutType> CreateFromFolder ( string folderPath )
    {
        if (String.IsNullOrEmpty(folderPath))
            throw new ArgumentNullException("folderPath", "Folder path must not be null");

        var userLayoutXmlFilePaths = Directory.GetFiles ( folderPath ).Where ( filePath => filePath.EndsWith ( ".UserLayout.xml", StringComparison.InvariantCultureIgnoreCase ) );
        return userLayoutXmlFilePaths.Select(filePath => UserLayoutFactory.CreateFromFile(filePath));
    }

    public static UserLayoutType CreateFromFile ( string filePath )
    {
        using ( var stream = new StreamReader ( filePath ) )
        {
            return ( UserLayoutType ) new XmlSerializer ( typeof ( UserLayoutType ) ).Deserialize ( stream );
        }
    }

Anybody have any idea what is happening? See image below:

Answer 1

The main, probable, cause for why you're seeing new objects from this is that the IEnumerable<T> is wrapping a generator, and not a materialized collection.

Here's a simple LINQPad program to demonstrate:

void Main()
{
    IEnumerable<string> collection =
        from index in Enumerable.Range(1, 10)
        select "Index=" + index;

    var list1 = collection.ToList();
    var list2 = collection.ToList();

    ReferenceEquals(list1[0], list2[0]).Dump();
}

This will print False .

It will do this because the act of enumerating over the collection ( .ToList() in this case) will execute the deferred LINQ query, and since we're enumerating the collection twice, we execute it twice, producing different instances with the same values.