Two way Binding in WPF does not work with static members

Question

Matt Hamilton told me an interesting fact about WPF: binding in two way mode with a static variable is possible in version 4.5. Unfortunately V4.5 ist still beta, I decided to change my code to get my app finally run correct.

But - still I have similar problems, here we go:

I have a very simple class 'RecallConnectionSettings'. This member of this class should be accessible from everywhere in the code, so I decided to make them static (like this):

public class RecallConnectionSettings
    {
            private static string Server {get;set;} 
    }

As you can see: there is only one variable 'Server'. Now what I want is to make 2WayMode binding from a TextBox Text-property to that 'Server' value.

So I tried this:

<UserControl....>
    <UserControl.Resources>
            <local:RecallConnectionSettings x:Key="recallConf"/>
    </UserControl.Resources>
    <TextBox Text="{Binding Source={StaticResource recallConf}, Path=Server,  
Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" ... Name="txtServerAdress" />
</UserControl>

This works great when I change the value in the textbox - but not from the other side. If I change the 'Server' value (by hand), the text-property in my textbox will not update.

Of course not - as I now know I have to implement INotifyProperty in my RecallConnectionSettings-class. Then it looks like this:

 public class RecallConnectionSettings : INotifyPropertyChanged
    {
    public  event PropertyChangedEventHandler PropertyChanged;
    private static string s_server; 

    public static string Server
            {
                get { return s_server; }
                set
                {
                    s_server = value;
                    OnPropertyChanged("Server");
                }
            }

public static event PropertyChangedEventHandler PropertyChanged;



protected static void OnPropertyChanged(string name)
            {
                PropertyChangedEventHandler handler = PropertyChanged;
                if (handler != null)
                {
                    PropertyChanged(this, new PropertyChangedEventArgs(name));
                }
            }
    }

Well - this can't work too. Because there are only static methods, I can't use the class instance to call the event:

PropertyChanged(this, new PropertyChangedEventArgs(name));

So - what to do now? I thought about using a singleton, so I did this:

public class RecallConnectionSettings : INotifyPropertyChanged
    {
        private static RecallConnectionSettings instance;

        private RecallConnectionSettings(){}

        public static RecallConnectionSettings Instance
        {
            get
            {
                if(instance == null)
                {
                    instance = new RecallConnectionSettings();
                }
                return instance;
            }
        }
// ... here comes the other stuff
}

To make it work, I also have to prepare my UserControl, so I did this:

...    
<UserControl.DataContext>
            <local:RecallConnectionSettings/>
</UserControl.DataContext>
...

At this point there is no need to go on trying, because for doing this, the default constructor must be public.

No matter what I am doing: it does not work. Seems to me that I still do not understand how that works - would you be so kind and show me the trick ?

Answer 1

Keep the singleton solution and replace this:

...    
<UserControl>
    <UserControl.DataContext>
        <local:RecallConnectionSettings/>
    </UserControl.DataContext>
    ...
</UserControl>
...

By this:

...    
<UserControl DataContext="{x:Static local:RecallConnectionSettings.Instance}">
   ...
</UserControl>
...

Answer 2

WOW - thanks Nicolas, that works !

For the other readers - here is what you have to code for the textbox now:

<TextBox Text="{Binding Path=Server, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"  Name="txtServerAdresse"/>