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>
This code (inspired from DefaultValueConverter.cs @ referencesource.microsoft.com) works for a two way binding to a TextBox or similar control, as long as the FormatString leaves the ToString() version of the source property in a state that can be converted back.
(i.e. format like "#,0.00" is OK because "1,234.56" can be parsed back, but FormatString="Some Prefix Text #,0.00" will convert to "Some Prefix Text 1,234.56" which can't be parsed back.)
XAML:
<TextBox>
<TextBox.Text>
<MultiBinding Converter="{StaticResource ToStringFormatConverter}"
ValidatesOnDataErrors="True" NotifyOnValidationError="True" TargetNullValue="">
<Binding Path="Property" TargetNullValue="" />
<Binding Path="PropertyStringFormat" Mode="OneWay" />
</MultiBinding>
</TextBox.Text>
</TextBox>
Note duplicate TargetNullValue if the source property can be null.
C#:
/// <summary>
/// Allow a binding where the StringFormat is also bound to a property (and can vary).
/// </summary>
public class ToStringFormatConverter : IMultiValueConverter
{
public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture)
{
if (values.Length == 1)
return System.Convert.ChangeType(values[0], targetType, culture);
if (values.Length >= 2 && values[0] is IFormattable)
return (values[0] as IFormattable).ToString((string)values[1], culture);
return null;
}
public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture)
{
var targetType = targetTypes[0];
var nullableUnderlyingType = Nullable.GetUnderlyingType(targetType);
if (nullableUnderlyingType != null) {
if (value == null)
return new[] { (object)null };
targetType = nullableUnderlyingType;
}
try {
object parsedValue = ToStringFormatConverter.TryParse(value, targetType, culture);
return parsedValue != DependencyProperty.UnsetValue
? new[] { parsedValue }
: new[] { System.Convert.ChangeType(value, targetType, culture) };
} catch {
return null;
}
}
// Some types have Parse methods that are more successful than their type converters at converting strings
private static object TryParse(object value, Type targetType, CultureInfo culture)
{
object result = DependencyProperty.UnsetValue;
string stringValue = value as string;
if (stringValue != null) {
try {
MethodInfo mi;
if (culture != null
&& (mi = targetType.GetMethod("Parse",
BindingFlags.Public | BindingFlags.Static, null,
new[] { typeof(string), typeof(NumberStyles), typeof(IFormatProvider) }, null))
!= null) {
result = mi.Invoke(null, new object[] { stringValue, NumberStyles.Any, culture });
}
else if (culture != null
&& (mi = targetType.GetMethod("Parse",
BindingFlags.Public | BindingFlags.Static, null,
new[] { typeof(string), typeof(IFormatProvider) }, null))
!= null) {
result = mi.Invoke(null, new object[] { stringValue, culture });
}
else if ((mi = targetType.GetMethod("Parse",
BindingFlags.Public | BindingFlags.Static, null,
new[] { typeof(string) }, null))
!= null) {
result = mi.Invoke(null, new object[] { stringValue });
}
} catch (TargetInvocationException) {
}
}
return result;
}
}
Best Answer
According to http://codingcontext.wordpress.com/2008/11/17/headerformatstring-and-contentformatstring/, it looks like the
HeaderStringFormat
property isn't meant to be used with string format binding, but rather to specify the format to use when binding to an object that implementsIFormattable
.Given that, I couldn't get string formatting to work directly in the binding expression, either, so that might just be a bug. You should try notifying Microsoft and maybe they'll fix it.
With your workaround, I would suggest using a
TextBlock
rather than aTextBox
, since you probably don't want the user to be able to edit the text in theExpander
header.