Lubo Blagoev's Blog

PageFunctions are very interesting concept of WPF. If you want to use some kind of a navigation in your applications you will definitely have a scenario where you want to redirect the user to a page where to ask for some input and after that receive that in the previous page. This is where PageFunction can be used. It works the same as a normal functions. You call a function passing some parameters and expect a specific type of result. PageFunction<T> essentially allows you to treat a page navigation like a function call, in which a page navigates to (calls) a page function and expect a return result.

Though it is ok to use these types of functions in Visual Studio 2008, in Expression Blend you receive  these errors when you try to open a PageFunction in the designer:

The name "PageFunction" does not exist in the namespace
"http://schemas.microsoft.com/winfx/2006/xaml/presentation".

The property "TypeArguments" does not exist in the 
"http://schemas.microsoft.com/winfx/2006/xaml" namespace.

Recently Microsoft released Expression Blend 1 SP1 and Blend 2 December Preview. I was curious if these new builds supports page functions and I have posted a question on the forums (link). Interestingly enough I received answers that PageFunctions are not even supported in Blend v2. WHAT THE...? Man I can't wait till v4. I guess many of you either. One thing caught my eye on the experts answers:

No, we don't plan to support generics at the root of markup (of which
PageFunction<T> is an example) in version 2 either.

Well no support for generics at the root of xaml markup. If this is the problem can we workaround it? That's the million dollar question don't you think. The answer is YES we can. Below I will show you how.

First you need to have a PageFunction in your project. If you do not have one use Visual Studio and Add New Item to add new PageFunction with meaningful name. This is how xaml looks like by default. Expression Blend have a problem with this "x:TypeArguments" attribute and the with the root element "PageFunction"

<PageFunction
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:sys="clr-namespace:System;assembly=mscorlib" 
    x:Class="UntitledProject1.SelectUserNamePageFunction"
    x:TypeArguments="sys:String"
    Title="SelectUserNamePageFunction">
  <Grid>

  </Grid>
</PageFunction>

namespace UntitledProject1
{
    /// <summary>
    /// Interaction logic for SelectUserNamePageFunction.xaml
    /// </summary>
    public partial class SelectUserNamePageFunction : SelectUserNamePageFuncitonBase
    {
        public SelectUserNamePageFunction()
        {
            InitializeComponent();
        }
    }
}

namespace UntitledProject1
{
    /// <summary>
    /// Interaction logic for SelectUserNamePageFunction.xaml
    /// </summary>
    public partial class SelectUserNamePageFunction : SelectUserNamePageFuncitonBase
    {
        public SelectUserNamePageFunction()
        {
            InitializeComponent();
        }
    }       public class SelectUserNamePageFuncitonBase : PageFunction<string> { } 
}

<z:SelectUserNamePageFuncitonBase
    xmlns:z="clr-namespace:UntitledProject1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:sys="clr-namespace:System;assembly=mscorlib" 
    x:Class="UntitledProject1.SelectUserNamePageFunction"
    Title="SelectUserNamePageFunction">
  <Grid>

  </Grid>
</z:SelectUserNamePageFuncitonBase>

Happy coding!