Re: Question For List Generics - C# Discussion Boards

13-Jun-09 22:54

Well I jast have this to classes, I'll write it:

public delegate List< T > LoadChildHandler< T >(int parentId);

public class ObjectBase< T >
{
public int ID { get; set; }

public LoadChildHandler< object > LoadChild;
}

public class Country : ObjectBase< Country >
{

public string CountryName { get; set; }

public List< City > Cities
{
get
{

return (City)LoadChild(ID); <--- Error
}
}
}

Error: Cannot convert type 'System.Collections.Generic.List<object>' to 'G1_OnlineStoreObjects.City'

How can i solve this problem?? advise plz...

C# Developer

0x3c013-Jun-09 23:22

13-Jun-09 23:22

You're trying to cast the objects inside the List, but you're actually casting the List itself. If you're using version 3.5 of the .Net Framework, then you can simply use return LoadChild(ID).Cast<City>();

Between the idea
And the reality
Between the motion
And the act
Falls the Shadow

Sider8913-Jun-09 23:36

13-Jun-09 23:36

WELL i have done LoadChild(ID).Cast< City >();

but Error occurs aggain: Cannot implicitly convert type 'System.Collections.Generic.IEnumerable<g1_onlinestoreobjects.city>' to 'System.Collections.Generic.List< G1_OnlineStoreObjects.City >'. An explicit conversion exists (are you missing a cast?)

C# Developer

0x3c014-Jun-09 0:57

14-Jun-09 0:57

This is because the extension method Cast() returns an IEnumerable<t>. Just cast it to the property's type and you're done

Between the idea
And the reality
Between the motion
And the act
Falls the Shadow

Sider8914-Jun-09 1:04

14-Jun-09 1:04

I did not understand

right that: (City)LoadChild(ID).Cast< City >();

but occuers Error again

C# Developer

0x3c014-Jun-09 1:36

14-Jun-09 1:36

LoadChild(ID).Cast<City>() as List<City>;
Besides that, you seem to have a small amount of confusion with List casting. When you cast a List to a City, you do not cast the elements in the List, only the List itself. In order to cast the elements inside the List, you need to use the Cast() method. This method returns an IEnumerable; because List implements IEnumerable, you can cast the List. This does not cast the elements, just the containing class

Between the idea
And the reality
Between the motion
And the act
Falls the Shadow

Sider8914-Jun-09 5:30

14-Jun-09 5:30

well it's works but there is some another problem like this:
Here is class where I invoked function into delegate

public class CountryDAO : BaseDAO< Country >
{

public override Country Load(int id) //I try to do lazyloading
{
Country country = base.Load(id); //just loads from database to Country class objects
if (country != null)
{
country.LoadChild = LoadCity;// here I invoked it and here occurs error
}
return country;
}

private List< City > LoadCity(int parentId)
{
CityDAO cDao = new CityDAO();
return cDao.LoadList("CountryID=" + parentId.ToString());
}

}

Error: 'System.Collections.Generic.List< G1_OnlineStoreObjects.City > G1_OnlineStoreDAO.CountryDAO.LoadCity(int)' has the wrong return type

what's the matter, why cann't compile it?
I repeat it what you said me to cast bu it is not works again Frown | :(

C# Developer

0x3c014-Jun-09 5:47

14-Jun-09 5:47

This is because you have the wrong return type. The delegate is of type List<Country> LoadChild(int id), but the delegate you try to convert it to is of type List<City> LoadChild(int id). That's the main problem, but it seems more architectural than syntactic. Either your delegate type is wrong, City does not inherit from Country, or City and Country don't implement a common interface. Personally, I wouldn't use a generic delegate at all; I'd go for something like this:

public delegate List<City> LoadCitiesDelegate(int countryId);

public class CountryDAO : BaseDAO<Country>
{
    public override Country Load(int id)
    {
        Country country = base.Load(id);

        if(country != null)
            country.LoadChild = loadCity;
        return country;
    }

    private List<City> LoadCities(int countryId)
    {
        CityDAO cDao = new CityDAO();

        return cDao.LoadList("CountryID=" + countryId.ToString());
    }
}

This probably wouldn't compile, but it's a general idea of the way I would personally do it

Between the idea
And the reality
Between the motion
And the act
Falls the Shadow

Sider8914-Jun-09 19:39

Closing and recreating a form

14-Jun-09 19:39

Maybe it'll work, but in my project I have many classes and Functions with generics and it's almost impossible to change all Big Grin | :-D

I think I solve this like this, I write it:

public delegate List< T > LoadChildHandler< T >(int parentId);

public class ObjectBase< T >
{
public int ID { get; set; }
}

public class Country : ObjectBase< Country >
{

public LoadChildHandler< City > LoadChild; // I just declare delegate in child class and there are not resist between City and Country, well works fine

public string CountryName { get; set; }

public List< City > Cities
{
get
{
return LoadChild(ID);
}
}
}

and so all I want it to do in a different way...
Now I must dclare this delegate in other classes too, it would be fine to do solve this problem, without declaring in other classes and not such torture Smile | :)

C# Developer

SimpleData13-Jun-09 22:43

13-Jun-09 22:43

Hi I have a Windows form called mainwindow in my project. After it completes what it does, it closes and then recreates itself. But as I have noticed lately it recreates but doesnt't close itself.

Here is my code:

private void RestartAll()
{
   // These are not that important, only to transfer the same properties of the form to the new form.
   depo.multipleformlocation=this.Location;
   depo.multipleform = true;
   depo.ekrangoruntusuadedi = 0;

   this.Hide();

   mainwindow _mainwindow = new mainwindow();
   _mainwindow.ShowDialog();

   this.Close();

   // Here is my temporary solution (putting old form's thread to sleep infinitely because it shouldn't keep doing what it does)
   Thread.Sleep(Timeout.Infinite);
}

How can I close(destroy) the old form completely and recreate that form? When I don't completely close that old form, because I do this closing and recreating process more than once my App's memory usage gets much higher.

Thanks in advance.

Henry Minute14-Jun-09 0:14

14-Jun-09 0:14

I am having a little difficulty in understanding where the cause of your problem might lie.

Just to clarify things, what class contains the RestartAll method in the code you posted?

Henry Minute

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“I want to report a fraud. The government is lying to us all.”

SimpleData14-Jun-09 0:15

14-Jun-09 0:15

RestartAll method is in the mainform class.

Henry Minute14-Jun-09 0:31

14-Jun-09 0:31

Try this:

private void RestartAll()
{
   // These are not that important, only to transfer the same properties of the form to the new form.
   depo.multipleformlocation=this.Location;
   depo.multipleform = true;
   depo.ekrangoruntusuadedi = 0;

   this.Hide();

   using (mainwindow _mainwindow = new mainwindow())
   {
      _mainwindow.ShowDialog();
   }

   this.Close();

   // Here is my temporary solution (putting old form's thread to sleep infinitely because it shouldn't keep doing what it does)
   Thread.Sleep(Timeout.Infinite);
}

Using the using construct 'should' ensure that _mainwindow is disposed.

SimpleData14-Jun-09 3:05

14-Jun-09 3:05

Thank you. It seems to be working. Should I keep using the Thread.Sleep(Timeout.Infinite); line?

Henry Minute14-Jun-09 3:13

14-Jun-09 3:13

Try commenting it out and see if everything still works. As you said in your OP it was a temporary solution, and shouldn't be necessary. However, if you still have the old problem after commenting it, the fix hasn't worked and it's back to the drawing-board.

SimpleData14-Jun-09 3:36

14-Jun-09 3:36

I have commented it and seems like it's still working.

Thank you for your help.

Henry Minute14-Jun-09 3:40

14-Jun-09 3:40

Excellent.

Try to remember the 'using' construct, whenever you create anything IDisposable. I don't always, particularly when I'm 'just trying something out' and forgetting nearly always causes me problems.

Good Luck! Smile | :)

Luc Pattyn14-Jun-09 4:14

Luc Pattyn

14-Jun-09 4:14

Hi,

I don't understand what it is you are doing here. Either this code runs on the GUI thread, and your Sleep(Infinite) hangs it forever, or it runs on some other thread, and it is not allowed to touch any Controls.

I am afraid your approach is completely wrong.

Confused | :confused:

Luc Pattyn [Forum Guidelines] [My Articles]

DISCLAIMER: this message may have been modified by others; it may no longer reflect what I intended, and may contain bad advice; use at your own risk and with extreme care.

SimpleData14-Jun-09 4:43

14-Jun-09 4:43

Well, this method doesn't run on the GUI thread but Control.CheckForIllegalCrossThreadCalls is set to false, so all threads are allowed to touch all the Controls.

Luc Pattyn14-Jun-09 4:49

Luc Pattyn

14-Jun-09 4:49

SimpleData wrote:
so all threads are allowed to touch all the Controls.

That is absolutely false. All CheckForIllegalCrossThreadCalls=false; does is disable the checking (returning to the unhealthy situation that existed in .NET 1.0/1.1), it does not ALLOW anything. Controls are not thread-safe and should be touched only by the thread that created them, and since all Controls are linked somehow (through Control collections, Form Z-Order, etc) that basically means the initial/main/GUI thread is the only one that can touch them. Violating this rule may and will result in abnormal app behavior, typically the GUI freezing pretty soon (or after a possibly long while, but failure is guaranteed).

Smile | :)

Luc Pattyn [Forum Guidelines] [My Articles]

DISCLAIMER: this message may have been modified by others; it may no longer reflect what I intended, and may contain bad advice; use at your own risk and with extreme care.

SimpleData14-Jun-09 4:52

14-Jun-09 4:52

That's good to know, I didn't know that.

I will test my application's stability, maybe it won't be a problem. Smile | :)

Dave Kreskowiak14-Jun-09 4:58

Dave Kreskowiak

14-Jun-09 4:58

It may pass your tests, but when released into the real world, your code WILL fail. It's just a matter of time before someone out there find the combination of circumstances that will make it happen.

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Dave Kreskowiak
Microsoft MVP
Visual Developer - Visual Basic
2006, 2007, 2008

Dave Kreskowiak14-Jun-09 4:55

Dave Kreskowiak

14-Jun-09 4:55

Just because the property is there doesn't mean it should be used.

I concur with Luc, your approach is completely wrong and unsupportable. I really can't suggest a replacement because it isn't clear what you're trying to accomplish.

Also, when you show a form with ShowDialog, you are responsible for calling Dispose on it to free up unmanaged resources before the object goes out of focus and is destroyed. As it stands now, your code leaks system resources, specifically, window handles. Your code, if left running long enough, will eventually crash Windows.

A guide to posting questions on CodeProject[^]

Dave Kreskowiak
Microsoft MVP
Visual Developer - Visual Basic
2006, 2007, 2008

SimpleData14-Jun-09 5:07