WPF binding to another property's binding in a style

Question

I'm not sure the best way to ask this question (sorry for the ambiguous question title), but essentially I'd like to set the MaxLength property on a TextBox using a value converter that is passed in a property from the data context, and the property on the passed-in property as the converter parameter. I'd like to do all this in a style, as opposed to on a control-by-control basis. Here's an example of doing this in a non-styled manner:

<TextBox Text="{Binding MyPropertyName.TheirPropertyName}" MaxLength="{Binding MyPropertyName, Converter={StatocRespirceMyCoolConverter}, ConverterParameter=TheirPropertyName}" />

(In case you're wondering, TheirPropertyName represents a property on the type of MyPropertyName that has an attribute like [StringMaxLength(15)], which I'd be able to get to and return inside the value converter.) Additionally, is there any way to pass in the type of MyPropertyName as opposed to the instance? I only need the type to do the StringMaxLength attribute lookup.

Anyway, how could I go about doing something like this in a style? I've gotten as far as:

<Setter Property="MaxLength">
    <Setter.Value>
        <Binding Converter="{StaticResource textFieldMaxLengthConverter}" />
    </Setter.Value>
</Setter>

But that passes the overall datacontext in to the value converter, as opposed to the MyPropertyName object, and I really have no clue if I can have it parse the MyPropertyName.TheirPropertyName part of the binding to pass TheirPropertyName in on the ConverterParameter attribute of the binding.

Any guidance would be really appreciated!

Answer 1

you can pass in lutiple properties to your converter by using a multi binding, this allows you to do a binding on as may properties as you want, and if any of the properties change (i.e. implent INotifyPropertyChanged) the binding will be reevaluated. for what you are doing you would have to use reflection to find a property on the passed in object with a particular property name that matches your converter parameter. i dont think you will end up using the code below, but it shows you can have multiple parameters to your binding in xaml. including the path, converter, converter parameter. Im not sure about the relative source but however, but i think you might need it to do what you want. have a look at debugging Data Bindings for a good way to debug. this technique is essential. i use it continually.

  <Setter
     Property="MaxLength">
     <Setter.Value>
        <Binding
           Converter="{StaticResource textFieldMaxLengthConverter}"
           RelativeSource="{RelativeSource TemplatedParent}"
           Path="MyPropertyName"
           ConverterParameter="TheirPropertyName" />
     </Setter.Value>
  </Setter>

Answer 2

Ok, after some more digging, I've figured this out to my satisfaction. I'm binding to RelativeSource Self and then parsing the binding expression on the Text property (since this is a TextFieldMaxLength converter, I am presuming I'm working against a TextBox.

The styling up in the resource dictionary:

<Style TargetType="TextBox"> 
  <Setter Property="MaxLength">
    <Setter.Value>
      <Binding Converter="{StaticResource textFieldMaxLengthConverter}" RelativeSource="{RelativeSource Self}" />
    </Setter.Value>
  </Setter>
</Style>

The usage (basically showing nothing special needs to be done since it's all in the style):

<TextBox Text="{Binding MyPropertyName.TheirPropertyName}" />

The Convert Method for the textFieldMaxLengthConverter:

public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
  Control control = value as Control;
  BindingExpression be = control.GetBindingExpression(TextBox.TextProperty);
  if (be != null)
  {
    string boundPropertyName = be.ParentBinding.Path.Path;
    // .. boundPropertyName here is MyPropertyName.TheirPropertyname, do some parsing and return a value based on that
  }
}

(Obviously my actual implementation is a bit more complex/handles unexpected input/uses reflection as per my original question's statement).

Anyway, thought I would post this solution in case anyone else tries to do something similar, or if there might be a better way to do this than I am using.