Wpf binding problem

Question

I have in my window a rectangle with a tooltip, Clicking the button suppose to change the tooltip text but it doesn't.

XAML:

 <Grid>
    <Grid.RowDefinitions>
        <RowDefinition />
        <RowDefinition />
    </Grid.RowDefinitions>

    <Grid.Resources>
        <ToolTip x:Key="@tooltip">
            <TextBlock Text="{Binding Name}" />
        </ToolTip>
    </Grid.Resources>

    <Rectangle Width="200" Height="200" Fill="LightBlue" VerticalAlignment="Center" HorizontalAlignment="Center"
            ToolTip="{DynamicResource @tooltip}" />
    <Button Click="Button_Click" Grid.Row="1" Margin="20">Click Me</Button>
</Grid>

code behind:

public partial class Window1 : Window
{

    public Window1()
    {
        DataContext = new Person { Name = "A" };
        InitializeComponent();
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
        DataContext = new Person { Name = "B" };
    }
}

Community · Accepted Answer · 2020-06-20 09:12:55Z

3

Change binding to:

Example:

<ToolTip x:Key="@tooltip">
    <TextBlock Text="{Binding RelativeSource={RelativeSource Mode=Self}, Path=Parent.PlacementTarget.DataContext.Name}" />
</ToolTip>

Reason:

The reason this works and yours doesn't is because the TextBlock element never gets notified of a change in a property named Name that you bind to in your code.

To fix that, in this example, TextBlock is bound to Parent (ToolTip) > PlacementTarget (Button) > DataContext (Person) > Name property. Here TextBlock has reference to PlacementTarget which in your case is a Button. Button notifies TextBlock of a change in it's DataContext property value on which notification, TextBlock updates itself.

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Dec 21, 2010 at 9:30

decyclone

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2 Comments

Erez Over a year ago

1. Thanks!!! 2. Can you please explain why your solution works while mine don't? I really didn't understand.

David Over a year ago

the Text="{Binding Name}" in your code suggests the toolTip has its dataContext set to tan instance of the Person class, which is not the case here. decyclone's code sets the dataContext has needed for the binding to work.

biju · Accepted Answer · 2010-12-21 09:22:31Z

Yes it works but not consistently.It works sometime but not all times.Seems more to be a timing bug to me.But you can change your approach a little to get it working

First you have to implement notification mechanism in your Person class

Sample

 public class Person:INotifyPropertyChanged
{
    private string name;

    public string Name
    {
        get { return name; }
        set { name = value; OnPropertyChanged("Name"); }
    }



    #region INotifyPropertyChanged Members

    public event PropertyChangedEventHandler PropertyChanged;
    public void OnPropertyChanged(string name)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(name));
        }
    }
    #endregion
}

Then you need to Create a Property for your person class and set as your datacontext.Now you can just change the name value on your button click and it works well

Sample

public Window1()
    {
        MyPerson = new Person();
        MyPerson.Name = "A";
        DataContext = MyPerson;
        InitializeComponent();
    }

    private Person myPerson;

    public Person MyPerson
    {
        get { return myPerson; }
        set { myPerson = value; OnPropertyChanged("MyPerson"); }
    }
    private void Button_Click(object sender, RoutedEventArgs e)
    {
        MyPerson.Name = "B";
    }

See if this helps...

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