I just spent several hours on an issue when using VS2012, WPF 4.5 and design-time data, specifically the DesignInstance attribute.
Goal: I wanted to have design-time data support in my WPF project (MVVM-based), both in VS2012 and Blend, and I could not for the life of me make the MVVMLight approach work consistently.
So I tried to change to "just" using the built-in support for design-time data, using the markup extension provided with Blend.
Problem: Consider the following code:
<Window x:Class="Nova.View.AlertsView"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:local="clr-namespace:Nova.View"
xmlns:vm="clr-namespace:Nova.ViewModel"
mc:Ignorable="d"
DataContext="{Binding Alerts, Source={StaticResource Locator}}"
d:DataContext="{d:DesignInstance vm:DesignAlertsViewModel, IsDesignTimeCreatable=True}"
... />
Both VS2012 and Blend reports "the name DesignAlertsViewModel does not exist in the namespace clr-namespace:Nova.ViewModel", even though intellisense resolves it just fine, and you have checked a thousand times that the namespace and class name are both correct.
I ran into this error myself in VS2013 then found this question. After searching more, I found an answer that helped me solve it.
instead of
d:DataContext="{d:DesignInstance vm:DesignAlertsViewModel, IsDesignTimeCreatable=True}"
use
d:DataContext="{d:DesignInstance d:Type=vm:DesignAlertsViewModel, IsDesignTimeCreatable=True}"
I was using "...d:DesignInstance Type=vm:..." and that displays the same error described above. As soon as I add "d:" everything works.
Here is the thread I found that helped.