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

UPDATE: There is an alternate technique described here and example code available from GoogleCode.

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.

Finally here is an example of using MultiBinding with StringFormat.