WPF User Control's DataContext is Null

Question

I have a user control where the XAML of the control can bind to the appropriate properties from the parent's data context like normal (the data context propagates in xaml).

For example, I have a window whose DataContext I am setting to ObjectA for example. My user control within the window is then try to access the properties within the dataContext

So my window's xaml and code behind can both see a non-null DataContext.

My control that DataContext propagates to can see a non-null DataContext in the Xaml but not in the code behind.

What is the proper way of handling this?

Bijington · Accepted Answer · 2008-11-18 16:20:53Z

32

failing that if you need to check whether the DataContext is being set you can use the DataContextChanged

public partial class UserControl1 : UserControl
{
    public UserControl1()
    {
        InitializeComponent();

        DataContextChanged += new DependencyPropertyChangedEventHandler(UserControl1_DataContextChanged);
    }

    void UserControl1_DataContextChanged(object sender, DependencyPropertyChangedEventArgs e)
    {
        // You can also validate the data going into the DataContext using the event args
    }
}

Note it wont enter UserControl1_DataContextChanged until DataContext is changed from null to a different value.

Not sure if this answers your question but can be quite handy to use in debugging issues.

answered Nov 18, 2008 at 16:20

Bijington

3,7715 gold badges38 silver badges55 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Darlene Over a year ago

I had to subscribe to the DataContextChanged event before InitializeComponent so that I could handle the event the first time DataContext was set.

Skystrider Over a year ago

Also works to do: DataContextChanged += UserControl1_DataContextChanged;

Jobi Joy · Accepted Answer · 2008-11-17 19:35:41Z

17

I think you are checking the 'DataContext' in the constructor of the UserControl. It will be null at the Constructor since the user control hasnt yet created while execution is in the constructor code. But check the property at Loaded event you will see the object properly.

public partial class UserControl1
{
    public UserControl1()
    {
        this.InitializeComponent();

        //DataContext will be null here 
        this.Loaded += new RoutedEventHandler(UserControl1_Loaded);
    }

    void UserControl1_Loaded(object sender, RoutedEventArgs e)
    {
        //Check DataContext Property here - Value is not null
    }
}

answered Nov 17, 2008 at 19:35

Jobi Joy

50.3k20 gold badges111 silver badges120 bronze badges

2 Comments

Pakk Over a year ago

Not sure if anything's changed since this answer - but Loaded event has a context of NULL.

blearyeye Over a year ago

In my program (.NET Framework 4.6.2), DataContext is in fact initialized after InitializeComponent executes.

hughdbrown · Accepted Answer · 2009-02-16 22:11:30Z

14

I would check to see whether you are having a binding error at runtime. Add this namespace to your XAML:

xmlns:debug="clr-namespace:System.Diagnostics;assembly=System"

and check the debugger's Output window for relevant error messages.

Alternatively, can you show us more code?

answered Feb 16, 2009 at 22:11

hughdbrown

49.3k20 gold badges89 silver badges111 bronze badges

3 Comments

Jason Stevenson Over a year ago

Thank you... this is sweet... I wish I had found this weeks ago... lol.

BeardinaSuit Over a year ago

Oh well. Solved my completely-unrelated-problem too :) Thanks.

Jim Over a year ago

Teach a man to fish... I also had no idea you could do this. Thanks.

Collectives™ on Stack Overflow

WPF User Control's DataContext is Null

3 Answers 3

2 Comments

2 Comments

3 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

2 Comments

3 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related