Hi all
As you all remember in my article about Custom WPF Context Menu I mentioned that my WPF Binding Converter was a singleton, and I promised to tell you why, so here comes the 3 ways I know of using WPF Binding Converters. We will start from the worst (in my opinion) and move on the the best.
To start off here is the Binding Converter:
2: {
3: #region IValueConverter Members
4:
5: public object Convert(object value,
6: Type targetType,
7: object parameter,
8: System.Globalization.CultureInfo culture)
9: {
10: if (value is int)
11: {
12: int a = (int)value;
13: switch (a)
14: {
15: case 1:
16: return "One";
17: case 2:
18: return "Two";
19: case 3:
20: return "Three";
21: default:
22: return "Bigger than Three";
23: break;
24: }
25: }
26: return "Error";
27: }
28:
29: public object ConvertBack(object value,
30: Type targetType,
31: object parameter,
32: System.Globalization.CultureInfo culture)
33: {
34: return null;
35: }
36:
37: #endregion
38: }
Pretty simple.
The Static Resource Way
This is the most simple way of using WPF converters, create you Converter Class and link to it as a StaticResource. Here is the code:
2: <conv:IntConverter x:Key="IntConverter"></conv:IntConverter>
3: </Window.Resources>
4: <StackPanel>
5: <TextBlock x:Name="Result" Margin="15" FontSize="20"
6: HorizontalAlignment="Center" VerticalAlignment="Center"
7: Text="{Binding Converter={StaticResource IntConverter}}">
8: </TextBlock>
9: </StackPanel>
10:</Window>
This is very easy and simple but there is one drawback to it which I hope the WPF team will fix someday. It appears that every time there is a need to use the converter the Application will create a new Converter class, use it and then dispose it. If you have a heavy application with allot of binding and converters this could be costly. I am not 100% sure that it is true but I did allot of checking and the performance is slower. So lets move on to the next way.
The Static Converters Class
Here we will not allow WPF to create and dispose the Converters all the time be creating a Static Class that will hold all the Converters in one place something like this:
2: {
3: public static readonly IntConverter m_IntConverter = new IntConverter();
4: }
You can add as may converters as you want to this class. It is sort of a Converters repository. Here is the code for using the converter:
2: <TextBlock x:Name="Result" Margin="15" FontSize="20" HorizontalAlignment="Center"
3: VerticalAlignment="Center"
4: Text="{Binding Converter={x:Static conv:ConvertersHolder.m_IntConverter}}">
5: </TextBlock>
6: </StackPanel>
Ok, lets take a break.
The first example is simple but costly. the second one is better in performance but requires maintaining a Converters class. If only we could get rid of maintaining the Converters Class… We Can!!!
The Singleton Converter Class
This method will allow us to enjoy both worlds. First we will need to add a singleton implementation to the Converter ( I know it is not thread safe and all…):
2:
3: private static IntConverter instance = new IntConverter();
4:
5: private IntConverter()
6: {
7: }
8:
9: /// <summary>
10: /// The converter instance
11: /// </summary>
12: public static IntConverter Instance
13: {
14: get
15: {
16: return instance;
17: }
18: }
19:
20: #endregion
And the Usage is similar to the second example:
2: <TextBlock x:Name="Result" Margin="15" FontSize="20"
3: HorizontalAlignment="Center"
4: VerticalAlignment="Center"
5: Text="{Binding Converter={x:Static conv:IntConverter.Instance}}">
6: </TextBlock>
7: </StackPanel>
No you don’t have to maintain a converter class. I think this is the best method for using Converters. I made myself a template for a converter that already has the singleton implementation so creating a new one is easy.
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Don’t forget to check out the Context Menu article where it all started 
Need help or know of a different way of using Converters? Let me know
Amit.
By contrapunctus on Jul 18, 2008 | Reply
very nice! thanks for this idea.
By The Reddest on Jul 20, 2008 | Reply
We’ve tested it, and the converter is not created and destroyed every time it’s used. If it where, you would not be able to persist any information between convert calls, which would not be good. Regardless of how many times the the converter is referenced in xaml or how many times the converter is used, it is only instantiated once when it is added to your resources collection.
By Amit on Jul 21, 2008 | Reply
@ Reddest
Really?
How did you test it? My tests showed different results. Maybe I should conduct them again, they were a long time ago…
Amit
By The Reddest on Jul 21, 2008 | Reply
We did a simple Console.WriteLine() in the converter’s constructor. We then tested persistence by updating a member variable every time convert was called. Both indicated the object was created once. I’d be curious to how you tested it.
By Amit on Jul 22, 2008 | Reply
I must say I did the something very much alike.
I will have to test it again, thanks!
Will get back once tested.
Amit