Using Singleton Pattern in iComparer Interface Class

[RileyCat]

I am writing a C# Windows app that will contain more than one ListView control. Because one form will contain more than one ListView control, I want to try to use the Singleton pattern on the implementation of my "iComparer" interface class. I am developing in VS2003

The code compiles nicely, but at run time, I get the error of "Object Is Not Set To Instance Of An Object".

Is there anything different I need to be doing to assign the singleton instance of my "iComparer" class to the ListViewItemSorter?

Or, Is there some reason why I cannot make this a singleton and if so, how else can I define the "iComparer" class so that there is only one instance alive throughout the life of the application.

I am attaching code files for reference, but here is some code snippets, too .......

From "Form1.CS"

ListViewColumnSorter.GetInstance() ;
this.listBox1.ListViewItemSorter = (IComparer)(ListViewColumnSorter.GetInstance()) ;

From "ListViewColumnSorter.cs"

public sealed class ListViewColumnSorter : IComparer
{
/// <summary>
/// ALBREK01 :: Define static singleton instance of class
/// </summary>
private static readonly ListViewColumnSorter instance = new ListViewColumnSorter() ;

// Explicit static constructor to tell C# compiler
// not to mark type as beforefieldinit
// ALBREK01 :: Class constructor
static ListViewColumnSorter()
{
InitColumnSorter () ;
}

// ALBREK01 :: Public access to singleton class instance
public static ListViewColumnSorter GetInstance()
{
return (instance) ;
}
}


Stay Cool Ya'll!

-- Kristin

 

[JurkMonkey]

Kristin,

I have to wonder why you chose a singleton for this class. The danger is if you have two different listviews and one of them needs to be sorted differently than the other. If 2 listviews require the same columnsorter then I would create a controller to work with both of those listviews directly.

By declaring a singleton - you can only ever have one instance in your application.

For the sake of a singleton, I have modified your code. Some may argue about this implementation but I have never had an issue with it.

The usage now changes too. What was:

ListViewColumnSorter.GetInstance() ;
this.listBox1.ListViewItemSorter = (IComparer)(ListViewColumnSorter.GetInstance()) ;

is now

this.listBox1.ListViewItemSorter = ListViewColumnSorter.GetInstance();

You should not need to cast in this case because your ListViewColumnSorter implements the IComparer interface.

public sealed class ListViewColumnSorter : IComparer
{
    private static ListViewColumnSorter instance = null;

   static ListViewColumnSorter()
   {
      InitColumnSorter () ;
   }

   public static ListViewColumnSorter Instance
   {
      get
      {
         if (instance == null)
         {
            instance = new ListViewColumnSorter();
         }

         return instance;
       }
   }
}

http://www.ubuntu.com/

 

[chrissie1]

This is a definite overuse of the singleton pattern.

Christiaan Baes
Belgium

My Blog

 

[Craig0201]

Agreed.

Ask the simple question......do I need to guarantee the instance only exists once?

The answer is no.

Hence no need for singleton.

C

 

[RileyCat]

Okay this is great! My next question is .... Do I need to destruct the instance of the comparer class as the form closes? Or does form dispose do that for me?

Stay Cool Ya'll!

-- Kristin

 

[Craig0201]

Depends!

Personally, I wouldn't. I could happily wait for it to be disposed by the form and garbage collected. Others might think otherwise.

C

 

[JurkMonkey]

You really only need to write a destructor if you are working with unmanaged memory such as images.

If you are concerned about memory issues you can implement the IDisposable interface.

http://www.ubuntu.com/

 

[chiph]

Download FxCop, or run CodeAnalysis (if you have the Team Developer edition). It'll tell you if you're missing an IDisposable implementation anywhere.

Chip H.


____________________________________________________________________
If you want to get the best response to a question, please read FAQ222-2244 first