WPF change Button Content on ViewModel.PropertyChanged event

Question

My attempt (below) fails:

<Canvas x:Key="Lock" ... />
<Canvas x:Key="Unlock" ... />

<Style x:Key="LockButtonStyle" TargetType="{x:Type Button}">
    <Style.Triggers>
        <DataTrigger Binding="{Binding Path=IsReadOnly}" Value="True">
            <DataTrigger.Setters>
                <Setter Property="Content" Value="{StaticResource Lock}" />                           
            </DataTrigger.Setters>
        </DataTrigger>
        <DataTrigger Binding="{Binding Path=IsReadOnly}" Value="False">
            <DataTrigger.Setters>
                <Setter Property="Content" Value="{StaticResource Unlock}" />
            </DataTrigger.Setters>
        </DataTrigger>
    </Style.Triggers>
</Style>

...

<Button Content="{StaticResource Lock}" />

I'm trying to get the button to change when the IsReadOnly property on the ViewModel changes (it fires INotifyPropertyChanged.PropertyChanged with "IsReadOnly" as the PropertyName). What am I missing?

.NET 3.5

ANSWER (at least for me - doesn't support the general case):

I just wrote a quick converter for boolean property binding.

[ValueConversion(typeof(bool), typeof(object))]
public class BooleanValueConverter : IValueConverter
{
    public object FalseValue { get; set; }
    public object TrueValue { get; set; }

    #region IValueConverter Members

    public object Convert(object value, Type targetType, 
                          object parameter, CultureInfo culture)
    {
        return (bool)value ? this.TrueValue : this.FalseValue;
    }

    public object ConvertBack(object value, Type targetType, 
                              object parameter, CultureInfo culture)
    {
        return object.Equals(this.TrueValue, value) ? true : false;
    }

    #endregion
}

...

<local:BooleanValueConverter x:Key="LockOrUnlock" 
    TrueValue="{StaticResource Unlock}" 
    FalseValue="{StaticResource Lock}" />

...

<Button Content="{Binding Path=IsReadOnly, 
                             Converter={StaticResource LockOrUnlock}}" />

Answer 1

If you set the property "Content" of the Button, you cannot change it with a trigger, because the first one takes precedence.
Try to remove the setting of Content and it should work, because triggers will do the right work themselves.