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Intermediate Materialization

If you are not careful, in some situations you can drastically alter the memory and performance profile of your application by causing premature materialization of collections in your queries. Some standard query operators cause materialization of their source collection before yielding a single element. For example, Enumerable.OrderBy first iterates through its entire source collection, then sorts all items, and then finally yields the first item. This means that it is expensive to get the first item of an ordered collection; each item thereafter is not expensive. This makes sense: It would be impossible for that query operator to do otherwise.

This example alters the previous example. The AppendString method calls ToList<TSource> before iterating through the source. This causes materialization.

NoteNote:

The following example uses the yield return construct of C#. Because there is no equivalent feature in Visual Basic 2008, this example is provided only in C#.

public static class LocalExtensions
{
    public static IEnumerable<string>
      ConvertCollectionToUpperCase(this IEnumerable<string> source)
    {
        foreach (string str in source)
        {
            Console.WriteLine("ToUpper: source >{0}<", str);
            yield return str.ToUpper();
        }
    }

    public static IEnumerable<string>
      AppendString(this IEnumerable<string> source, string stringToAppend)
    {
        // the following statement materializes the source collection in a List<T>
        // before iterating through it
        foreach (string str in source.ToList())
        {
            Console.WriteLine("AppendString: source >{0}<", str);
            yield return str + stringToAppend;
        }
    }
}

class Program
{
    static void Main(string[] args)
    {
        string[] stringArray = { "abc", "def", "ghi" };

        IEnumerable<string> q1 =
            from s in stringArray.ConvertCollectionToUpperCase()
            select s;

        IEnumerable<string> q2 =
            from s in q1.AppendString("!!!")
            select s;

        foreach (string str in q2)
        {
            Console.WriteLine("Main: str >{0}<", str);
            Console.WriteLine();
        }
    }
}

This example produces the following output:

ToUpper: source >abc<
ToUpper: source >def<
ToUpper: source >ghi<
AppendString: source >ABC<
Main: str >ABC!!!<

AppendString: source >DEF<
Main: str >DEF!!!<

AppendString: source >GHI<
Main: str >GHI!!!<

In this example, you can see that the call to ToList<TSource> causes AppendString to enumerate its entire source before yielding the first item. If the source were a large array, this would significantly alter the memory profile of the application.

Standard query operators can also be chained together. The final topic in this tutorial illustrates this.

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