WinRt :XAML Tag doesnot exist in the namespace - windows-store-apps

Question

I am creating a project to which I want to validate textbox ,therefore use prism for that and implementing example like this below
http://msdn.microsoft.com/en-us/library/windows/apps/xx130660.aspx
All I have implemented worked fine but when I changes the code in Page's xaml than it doesn't find the classes etc. which I have implemented .
For Example
<prism:VisualStateAwarePage
xmlns:prism="using:Microsoft.Practices.Prism.StoreApps"
xmlns:vm="using:PrismExample.ViewModels"
xmlns:Behaviors="using:PrismExample.Behaviors"
x:Class="PrismExample.Views.UserInfoView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:interactivity="using:Microsoft.Xaml.Interactivity"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006">
<Page.DataContext>
**<vm:UserInfoViewModel/>**
</Page.DataContext>
<TextBox x:Name="FirstNameValue" Grid.Row="2"
Text="{Binding UserInfo.FirstName, Mode=TwoWay}">
<interactivity:Interaction.Behaviors>
<!--<Behaviors:HighlightFormFieldOnErrors PropertyErrors="{Binding UserInfo.Errors[FirstName]}">-->
**<quickstartBehaviors:HighlightFormFieldOnErrors** PropertyErrors="{Binding UserInfo.Errors[FirstName]}" />
</interactivity:Interaction.Behaviors>
</prism:VisualStateAwarePage>
Now here ,the bold text gives the error that it doesnot exists in the namespace.
Can anybody help me out of this. How I can I get rid of this.

Answer 1

You have:
<Page.DataContext>
**<vm:UserInfoViewModel/>**
</Page.DataContext>
Should this be:
<prism:VisualStateAwarePage.DataContext>
<vm:UserInfoViewModel/>
</prism:VisualStateAwarePage.DataContext>

Answer 2

In your code, you reference quickstartBehaviors but you only have an XMLNS of Behaviors defined in your page. You should look where HighlightFormFieldOnErrors is defined.
Let's pretend that works. But, Noor, I am not sure how to say this. Validation on a control is different than validation in the view model. What I mean is, I think there might be a better way for your to consider. If you are interested, read this: http://blog.jerrynixon.com/2014/07/lets-code-handling-validation-in-your.html
Best of luck!