WPF Design Time Data Confusion

Question

Why is it that when I create some design time data for my MVVM View, I have to nest the xmlns twice to access the properties of the property?

Example:

       <d:DesignProperties.DataContext>
            <local:UserListViewModel>
                <local:UserListViewModel.Users xmlns:local="clr-namespace:ChatModule.ViewModels">
                    <local:User Name="Person 1" />
                    <local:User Name="Person 2" />
                    <local:User Name="Person 3" />
                    <local:User Name="Person 4" />
                    <local:User Name="Person 5" />
                    <local:User Name="Person 6" />
                </local:UserListViewModel.Users>
            </local:UserListViewModel>
       </d:DesignProperties.DataContext>

I already have a local property defined in an xmlns of the UserControl. But to access the User struct in my underlying class, I have to define it again within the Users property.

Why is that?

All this works fine... I can see my runtime data which is awesome. I've never done this before, so it's neat to be able to see it. I notice that I frequently get errors that go away when I compile... but come back again any time I change the xaml.

The property 'Name' was not found in type 'User'.
The property 'Name' was not found in type 'User'.
The property 'Name' was not found in type 'User'.
The property 'Name' was not found in type 'User'.
The property 'Name' was not found in type 'User'.
The property 'Name' was not found in type 'User'.

Obviously it works, or else I wouldn't be able to see the runtime data.

I also want to extract this design time data into another xaml class, and reference it within this class so I don't have the actual design time data located within the view itself... merely a reference to another xaml file.

And so I have done so... like this...

<UserListViewModel xmlns="clr-namespace:ChatModule.ViewModels">
    <UserListViewModel.Users xmlns:local="clr-namespace:ChatModule.ViewModels">
        <local:User Name="Person 1" />
        <local:User Name="Person 2" />
        <local:User Name="Person 3" />
        <local:User Name="Person 4" />
        <local:User Name="Person 5" />
        <local:User Name="Person 6" />
    </UserListViewModel.Users>
</UserListViewModel>

However, when I get rid of the codebehind I get the same errors mentioned above(which go away on a build), but also an error that says

Cannot set properties on propety elements. Line 2 Position 6.

This doesn't allow me to build unlike the other errors. Why is this? How do I fix it? I'm still getting used to this design time data stuff.

Hmmm... when I host this in a new xaml file like I did in the second part, I cannot have the second xmlns:local. That is what is causing the issue. Why is this needed in the actual view xaml, but not in this separate xaml file? Why does it work this way? — Cowman, Sep 03 '12 at 01:55
FYI, it's best to post your findings as an answer rather than updating the question. Thanks for the post. — McGarnagle, Sep 03 '12 at 02:05
Thanks. I wish I could still get the answer to why the The property 'Name' was not found in type 'User' errors still show up... but the bulk of the question is at least answered. I went ahead and moved my edit as an answer. I'm still very new to stackoverflow, so please forgive me. — Cowman, Sep 03 '12 at 02:09

Cowman · Accepted Answer · 2012-09-03T03:12:38.083

Okay so after fiddling around with it, as per the comment of mine above, I have figured it out. It apparently works differently than the view xaml, in that I do not need to specify the xmlns:local twice in order to access the properties of the property. I do not know why this is exactly.

Here's how my code looks now.

In my main xaml, I have this:

<Grid Name="LayoutRoot" d:DataContext="{d:DesignData Source=DesignTimeData/UserListViewData.xaml}">
    <ListView ItemsSource="{Binding Users}" ItemTemplate="{StaticResource UserTemplate}"  VerticalContentAlignment="Center" HorizontalContentAlignment="Stretch" Background="#505050" >
    </ListView>
</Grid>

And in the separate xaml file I have this:

<local:UserListViewModel xmlns:local="clr-namespace:ChatModule.ViewModels">
    <local:UserListViewModel.Users >
        <local:User Name="Person 1" />
        <local:User Name="Person 2" />
        <local:User Name="Person 3" />
        <local:User Name="Person 4" />
        <local:User Name="Person 5" />
        <local:User Name="Person 6" />
    </local:UserListViewModel.Users>
</local:UserListViewModel>

I still get the "The property 'Name' was not found..." in the backing xaml file, but no longer in the main xaml file of the view. I find this to be pretty elegant. And in this way... the view still remains COMPLETELY oblivious to the actual ViewModel type... whereas when I had it in the view xaml file like before, it had to know that information, even if it's only at design time.

Here's it working by the way... this is in Visual Studio's xaml editor.

enter image description here

Here's an example of the issue with the errors, that makes it a chore guessing game getting things right. No intellisense and stuff unless I rebuild to clear the errors until I mess up some syntax again. It still works just fine as Design Time Data, so I don't know what's up.

enter image description here

I gotta say, it's a little bit frustrating doing it this way. I'm trying to create another Design Time Data xaml file... and the scope of the objects keeps coming in and out... depending on whether those errors are present. I have to build every time I type something to see the intellisense that pops up. Anybody know why this is? — Cowman, Sep 03 '12 at 02:54
This is one of those frustrating situations with VS where you have to unload and reload the project. This kind of thing happens to me way too much and I wish there was a better way to fix it. — rookie1024, Jan 12 '15 at 02:04

WPF Design Time Data Confusion

1 Answers1