Question

I am trying to set the ViewModel as the DataContext of the View using the following XAML code:

<UserControl.DataContext>
    <local:MyViewModel />
</UserControl.DataContext>

<UserControl.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="/MA_Resources/MA_ResourceDictionary.xaml" />
        </ResourceDictionary.MergedDictionaries>

        <local:MyViewModel x:Key="myViewModel" x:Name="myVM" />
    </ResourceDictionary>
</UserControl.Resources>

But, I observe that the view-model constructor is called twice. I understand the the view-model is getting instantiated twice in XAML and that I should set the DataContext as the StaticResource from the Resources. However, I am not able to figure out how to set the DataContext with the StaticResource.
I tried the following but it’s giving an exception:

<UserControl .... DataContext="{StaticResource myViewModel}" >

Please help me figuring out what should be the appropriate XAML code for assigning the DataContext.

Answer 1

It is not possible to reference the static resource, if it is defined later in the xaml file. Therefore, you could do the following:

<UserControl.Resources>
    <ResourceDictionary>
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="/MA_Resources/MA_ResourceDictionary.xaml" />
        </ResourceDictionary.MergedDictionaries>

        <local:MyViewModel x:Key="myViewModel" x:Name="myVM" />
    </ResourceDictionary>
</UserControl.Resources>
<UserControl.DataContext>
    <StaticResourceExtension ResourceKey="myViewModel"/>
</UserControl.DataContext>

I wonder why you want do define the ViewModel as a static resource. Personally, I would prefer the instantiation in the setter of the DataContext.