Wpf – How to bind to another control’s property from a trigger

stylestriggerswpfxaml

In my particular case, I want to bind to the IsReadOnly property of a TextBox to set the Content property of a Button? They are both part of the same StackPanel.

I've tried doing it with a DataTrigger with a Binding to the ElementName of the TextBox and a Trigger using the TextBox name as the SourceName.

Any thoughts?

Best Answer

You need to specify the trigger as part of a style -- the Triggers collection on the Button itself can only contain event triggers. With that in mind, a DataTrigger works fine. However, there is a wrinkle: the value from the Trigger Setter won't overwrite a local Content property. So you have to set the default Content in the Style as well. Here's how it looks:

<Button>  <!-- Note no content set directly on button -->
  <Button.Style>
    <Style TargetType="Button">
      <Setter Property="Content" Value="You may write!!!" />  <!-- Here is the 'normal' content -->
      <Style.Triggers>
        <!-- Here is how we bind to another control's property -->
        <DataTrigger Binding="{Binding IsReadOnly, ElementName=textBox}" Value="True">
          <Setter Property="Content" Value="NO NO NO" />  <!-- Here is the 'override' content -->
        </DataTrigger>
      </Style.Triggers>
    </Style>
  </Button.Style>
</Button>

Related Solutions

Wpf – Can you bind a DataTrigger to an Attached Property

You need to wrap the property in parentheses:

<DataTrigger Binding="{Binding Path=(Validation.Errors).YourAttachedProperty,...

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Some research results for OneWayToSource...

Option # 1.

// Control definition
public partial class FlagThingy : UserControl
{
    public static readonly DependencyProperty IsModifiedProperty = 
            DependencyProperty.Register("IsModified", typeof(bool), typeof(FlagThingy), new PropertyMetadata());
}

<controls:FlagThingy x:Name="_flagThingy" />

// Binding Code
Binding binding = new Binding();
binding.Path = new PropertyPath("FlagIsModified");
binding.ElementName = "container";
binding.Mode = BindingMode.OneWayToSource;
_flagThingy.SetBinding(FlagThingy.IsModifiedProperty, binding);

Option # 2

// Control definition
public partial class FlagThingy : UserControl
{
    public static readonly DependencyProperty IsModifiedProperty = 
            DependencyProperty.Register("IsModified", typeof(bool), typeof(FlagThingy), new PropertyMetadata());

    public bool IsModified
    {
        get { return (bool)GetValue(IsModifiedProperty); }
        set { throw new Exception("An attempt ot modify Read-Only property"); }
    }
}

<controls:FlagThingy IsModified="{Binding Path=FlagIsModified, 
    ElementName=container, Mode=OneWayToSource}" />

Option # 3 (True read-only dependency property)

System.ArgumentException: 'IsModified' property cannot be data-bound.

// Control definition
public partial class FlagThingy : UserControl
{
    private static readonly DependencyPropertyKey IsModifiedKey =
        DependencyProperty.RegisterReadOnly("IsModified", typeof(bool), typeof(FlagThingy), new PropertyMetadata());

    public static readonly DependencyProperty IsModifiedProperty = 
        IsModifiedKey.DependencyProperty;
}

<controls:FlagThingy x:Name="_flagThingy" />

// Binding Code
Same binding code...

Reflector gives the answer:

internal static BindingExpression CreateBindingExpression(DependencyObject d, DependencyProperty dp, Binding binding, BindingExpressionBase parent)
{
    FrameworkPropertyMetadata fwMetaData = dp.GetMetadata(d.DependencyObjectType) as FrameworkPropertyMetadata;
    if (((fwMetaData != null) && !fwMetaData.IsDataBindingAllowed) || dp.ReadOnly)
    {
        throw new ArgumentException(System.Windows.SR.Get(System.Windows.SRID.PropertyNotBindable, new object[] { dp.Name }), "dp");
    }
 ....

Best Answer

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Wpf – Can you bind a DataTrigger to an Attached Property

Wpf – OneWayToSource binding from readonly property in XAML

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