Regex

For some unknown reason people use Regex.

This is a useful link with examples from O'Reilly: Regex 101

Constructors

It became apparent the other day that I'd either forgotten some stuff about constructors or never knew it in the first place.

This web page has a good explanation of all the issues: Constructors

For example, what I hadn't really considered is that:

public class MySimpleClass
{
    public MySimpleClass()
    {
    }
}

is converted by the compiler into:

public class MySimpleClass
{
    public MySimpleClass() : base()
    {
    }
}

which makes a bunch of stuff easier to understand.

Similarly 'this' can be used to invoke constructors in the same class:

public class MySimpleClass : MyBaseClass
{
    public MySimpleClass() : this(5)
    {
    }
    public MySimpleClass(int x) : base(x)
    {
    }
}

DataTemplateSelector

On our till roll I want to display each different category of product using a specific DataTemplate.

We start by defining the DataTemplates to be used in the usual way:

<DataTemplate x:Key="listBookingTemplate">...
<DataTemplate x:Key="listCCardTemplate">...
<DataTemplate x:Key="listCashTemplate">...

<DataTemplate x:Key="listDefaultTemplate">...

Then the DataTemplateSelector which is the link between the xaml DataTemplates and the C# class (TillRollTemplateSelector) that determines which template to use:

<ss:TillRollTemplateSelector 

 BookingTemplate="{StaticResource listBookingTemplate}" 

 CCardTemplate="{StaticResource listCCardTemplate}"

 CashTemplate="{StaticResource listCashTemplate}"  

 DefaultTemplate="{StaticResource listDefaultTemplate}" 

 x:Key="tillTemplateSelector" />

Then we need to wire up the template selector resource (tillTemplateSelector) to the ListBox that will display the data:

<ListBox Name="theTillRoll" Height="250" Width="330"

   ItemsSource="{Binding Source={StaticResource ViewModel}, Path=SaleItems}" 

   ItemTemplateSelector="{StaticResource tillTemplateSelector}"

   />

Finally, we define the C# class:

public class TillRollTemplateSelector : DataTemplateSelector

{

   public DataTemplate BookingTemplate { get; set; }

   public DataTemplate CashTemplate { get; set; }

   public DataTemplate CCardTemplate { get; set; }

   public DataTemplate DefaultTemplate { get; set; }

   public override DataTemplate SelectTemplate(object item, DependencyObject container)

   {

      if (item is BookingLineItem) return BookingTemplate;

      if (item is CashPaymentLineItem) return CashTemplate;

      if (item is CCardPaymentLineItem) return CCardTemplate;

      return DefaultTemplate;

   }

}

The object item passed to the SelectTemplate method is of the same type as bound to the ListBox via the ItemsSource.

MultiBinding

I wanted to control the IsEnabled property of a Button based on the properties of two other controls.
Specifically I wanted to only enable the LogIn button if both the Site combobox and UserName textbox had a value available.

I created a MultiBinding for the Button like this:

<Button.IsEnabled>

   <MultiBinding Converter="{StaticResource loginConverter}">

      <Binding ElementName="xSite" Path="SelectedIndex"/>

      <Binding ElementName="xUsername" Path="Text"/>

   </MultiBinding>

</Button.IsEnabled>

And a IMultiValueConverter like this:

public class LoginConverter : IMultiValueConverter

{

   public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)

   {

      // SelectedIndex of the ComboBox

      int selectedIndex = (int)values[0];

      // Length of Text in TextBox

      int selectedLength = ((string)values[1]).Length;

      // Only enable Button if both Combo and TextBox have data

      if (selectedIndex == -1 || selectedLength==0)

      {

         return false;

      }

      else

      {

         return true;

      }

   }

   public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)

   {

      throw new NotImplementedException();

   }

}

The array of object[] values are presented in the same order as the Bindings within the MultiBinding.

Forcing OnPropertyChanged to update the UI

Having used my new Func<T, T> friend (see Action<T> and Func<T, TResult>) the bound TextBox refused to show the revised value. Even though OnPropertyChanged("...") had been called the property's "getter" was not called by WPF.

I don't really understand why.

To get round this problem I added a FormattingConverter to the TextBox called DoNothingConverter. Its presence is sufficient to ensure the UI updates.

public class DoNothingConverter : IValueConverter
{
   public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
   {
      return value;
   }
   public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
   {
      return value;
   }
}

Action<T> and Func<T, TResult>

I wanted to pass a validation routine to my SetValue method that uses generics. So I used the Action delegate system.

protected void SetValue(T newValue, Action validate)
{
...
  validate.Invoke(newValue);
...
}

That worked fine until I wanted to change the value (for formatting purposes). So I moved to using the Func delegate system instead. Since I want the result to be of the same type as the passed argument I used <T, T> instead of <T, TResult>.

protected T SetValue(T newValue, Func validate)
{
...
  newValue = validate.Invoke(newValue);
...
}

WPF transparent windows and VNC

 

We've found that VNC doesn't transmit any info for transparent windows in a WPF app..

So.. when we use VNC for support.. we can't see :

 

The logon box

The popup menus

Any MessageBoxes

 

Not handy…. (not sure of the solution to this yet – might be something we can do with VNC.. might be a "support skin" change.