Question

In the code below, I am trying to execute a method, which returns a value, in another thread. However, it just DOES NOT work!!!

    public void main()
    {
       lstChapters.DataContext = await TaskEx.WhenAll(LoadChapters());
    }



   //CAN'T use  async in this function, it requires Task<> which
   //Error appears on the code inside []

   public [async Task<object>] Convert(object[] values, Type targetType,
     object parameter, System.Globalization.CultureInfo culture)
   {
      dictChapters data = await IQ_LoadQuranXML.LoadChapters(TypeIndex);
   }




    internal static async Task< IEnumerable<dictChapters>> LoadChapters()
    {
        var element = XElement.Load("xml/chapters.xml");

        Task < IEnumerable < dictChapters >>  something = (Task<IEnumerable<dictChapters>>)  await TaskEx.Run(delegate
        {

        IEnumerable<dictChapters> Chapters =  
           from var in element.Descendants("chapter")
           orderby var.Attribute("index").Value
           select new dictChapters
           {
              ChapterIndex = Convert.ToInt32(var.Attribute("index").Value),
              ChapterArabicName = var.Attribute("name").Value,
              ChapterType = var.Attribute("type").Value,
           };
            return Chapters;}
        );

        return something; //An ERROR on this line
     }

    //Overriding method which does not return IEnumerable type. And it accepts index as integer.
    internal static dictChapters LoadChapters(string chIdx = "0")
    {
        int chIdxInt = Convert.ToInt32(chIdx);
        List<dictChapters> Chapters = (List<dictChapters>)  LoadChapters(); // ERROR is on this line too
        return Chapters.ElementAt(chIdxInt - 1); //index of chapter in the element starts from 0
    }

The Error is:

Cannot implicitly convert type ‘System.Threading.Tasks.Task<System.Collections.Generic.IEnumerable<iq_main.dictChapters>>‘ to ‘System.Collections.Generic.IEnumerable<iq_main.dictChapters>‘. An explicit conversion exists (are you missing a cast?)

And the Other error is..

Cannot convert type ‘System.Threading.Tasks.Task<System.Collections.Generic.List<iq_main.dictChapters>>‘ to ‘System.Collections.Generic.List<iq_main.dictChapters>

When I cast “something” explicitly like return (IEnumerable<dictChapters>) something then at runtime, I get “InvalidCastException”.

Answer 1

Actually, you’ll be getting a runtime cast error earlier than that. The problem is your cast of the TaskEx.Run result. When you await something, the Task wrapper is removed.

public void main()
{
  lstChapters.DataContext = await LoadChapters();
}

internal static Task<List<dictChapters>> LoadChapters()
{
  return TaskEx.Run(delegate
  {
    var element = XElement.Load("xml/chapters.xml");
    IEnumerable<dictChapters> Chapters =  
        from var in element.Descendants("chapter")
        orderby var.Attribute("index").Value
        select new dictChapters
        {
          ChapterIndex = Convert.ToInt32(var.Attribute("index").Value),
          ChapterArabicName = var.Attribute("name").Value,
          ChapterType = var.Attribute("type").Value,
        };
    return Chapters.ToList();
  });
}

There are a few other problems with your code as well. Remember that enumerations like this are lazily executed. You probably want to return Chapters.ToList(); so that the XML parsing happens on the thread pool thread.