Understanding dependancy objects - wpf

Question

I'm totally lost with dependancy objects and binding. I often get things working without understanding why and how, this question is about knowing what should be happening.
I have a tiny user control with the following XAML
<Grid>
<Image Source="{Binding Icon}"></Image>
<TextBlock Text="{Binding Title}"></TextBlock>
</Grid>
My code behind has the following
public static readonly DependencyProperty IconProperty =
DependencyProperty.Register("Icon", typeof(Image), typeof(MenuItem));
public Image Icon
{
get { return (Image)GetValue(IconProperty); }
set { SetValue(IconProperty, value); }
}
public static readonly DependencyProperty TitleProperty =
DependencyProperty.Register("Title", typeof(String), typeof(MenuItem));
public string Title
{
get { return (string)GetValue(IconProperty); }
set { SetValue(IconProperty, value); }
}
My MainWindow is empty, other than a reference to this control and to the ResourceDictionary. In the MainWindow code behind, I set the DataContext in the constructor.
<Window x:Class="AppUi.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:loc="clr-namespace:AppUi.Control"
Title="">
//set up to Resource Dictionary - all binding and styling works fine :)
<loc:MenuItem Icon="{Binding MailIcon}" Title="{Binding MailTitle}"></loc:MenuItem>
In the ModelView for the MainWindow, I have the following 2 properties
private Image_mailIcon;
public Image MailIcon{
//inotifyproperty implementation
}
private string _mailTitle;
public string MailTitle{
//inotifyproperty implementation
}
My question is, in the UserControl, how do I do the binding? Since it's a user control within a MainWindow, and the MainWindow already has a datacontext, I think the UserControl will inherit the DataContext from the parent (From what I have read).
So, in my UserControl XAML, should I be binding to the MainWindow's Code Behind properties OR to the ViewModel properties?
In other words, should my UserControl be
<Grid>
<Image Source="{Binding MailIcon}"></Image>
<TextBlock Text="{Binding MailTitle}"></TextBlock>
</Grid>
OR
<Grid>
<Image Source="{Binding Icon}"></Image>
<TextBlock Text="{Binding Title}"></TextBlock>
</Grid>
Or, because I'm using a DataContext and the UserControl inherits, do I even need the Dependancy Properties at all?

Answer 1

You normally don't want to overwrite DataContext passed through visual tree so you can use either ElementName or RelativeSource binding inside UserControl to change binding context. The easiest way to achive this is give UserControl some name and use it ElementName binding
<UserControl ... x:Name="myUserControl">
<!-- ... -->
<Grid>
<Image Source="{Binding Icon, ElementName=myUserControl}"/>
<TextBlock Text="{Binding Title, ElementName=myUserControl}"/>
</Grid>
<!-- ... -->
</UserControl>
This way binding is DataContext independent. You can also create UserControl with assumption it will always work with only specific type of DataContext and then you just use Path from that view model type but then DataContext of that UserControl must always be of the view model it's designed for (mostly inherited through visual tree)
<UserControl ...>
<!-- ... -->
<Grid>
<Image Source="{Binding MailIcon}"/>
<TextBlock Text="{Binding MailTitle}"/>
</Grid>
<!-- ... -->
</UserControl>
I would also change type of Icon property from Image to ImageSource for example. You already have Image control inside your UserControl and you just want to bind its Source

Answer 2

in the UserControl, how do I do the binding? ... the UserControl will inherit the DataContext from the parent
That is correct, the UserControl will inherit the DataContext from the parent Window. Therefore you can data bind from the UserControl directly to the parent Window.DataContext. Please note that you would bind to whatever object has been set as the DataContext, regardless of whether that was the code behind or a separate view model class.
However, you don't have to data bind to the parent's DataContext object in this situation... you have other options. You could data bind to your own UserControl DependencyPropertys using a RelativeSource Binding like this:
<TextBlock Text="{Binding Title, RelativeSource={RelativeSource
AncestorType={x:Type YourPrefix:YourUserControl}}}" />
You could also name your UserControl and reference its properties like this:
<TextBlock Text="{Binding Title, ElementName=YourUserControlName}" />
While this example seems to be more concise, don't overlook the first example, as RelativeSource is a useful and powerful friend to have.
should I be binding to the MainWindow's Code Behind properties OR to the ViewModel properties?
That's your choice... what do you want or need to data bind to? you just need to know that a direct data binding will use the auto set DataContext value, so if you don't want to use that, then you can just specify a different data source for the Binding as shown above.
Finally, regarding the need to use DependencyPropertys... you only need to declare them if you are developing a UserControl that needs to provide data binding abilities.