Get underlying data type of binding

Question

How to get the underlying data type of a bound property?

For testing purposes I created a viewmodel 'Person' with a property 'Age' of type Int32, that is bound to a textbox's text property.

Is there something like ...

BindingOperations.GetBindingExpression(this, TextBox.TextProperty).PropertyType

or can this information only be retrieved by reflection?

myBinding.Source.GetType().GetProperty("Age").PropertyType

Edit: I have a custom textbox class, where I want to attach my own validationrules, converters ...

It would be great to get the information inside f.e. the 'load'-event of the textbox class.

Answer 1

if the property is bound to a specific datatype you need to set the value in the text box to a valid value for the property before it will update the viewmodel source

It seems that any validation errors stop the view model updating. Which I think is rubbish.

Answer 2

You could get the value inside the Convert method of a converter:

public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture) {
    value.GetType(); / *The bound object is here
}

XAML

Text="{Binding Age, Mode=TwoWay,Converter={StaticResource converterName}}"

Not sure where you need access to the type, but it is available at that level, if you needed to transform the value.

Answer 3

The only way I have found to do this is by using reflection. The following code gets the binded object. It also handles nested binding {Binding Parent.Value} and if a converter is present - it returns the converted value. The method also returns the item's type for the cases in which the item is null.

    private static object GetBindedItem(FrameworkElement fe, out Type bindedItemType)
    {
        bindedItemType = null;
        var exp = fe.GetBindingExpression(TextBox.TextProperty);

        if (exp == null || exp.ParentBinding == null || exp.ParentBinding.Path == null 
            || exp.ParentBinding.Path.Path == null)
            return null;

        string bindingPath = exp.ParentBinding.Path.Path;
        string[] elements = bindingPath.Split('.');
        var item = GetItem(fe.DataContext, elements, out bindedItemType);

        // If a converter is used - don't ignore it
        if (exp.ParentBinding.Converter != null)
        {
            var convOutput = exp.ParentBinding.Converter.Convert(item, 
                bindedItemType, exp.ParentBinding.ConverterParameter, CultureInfo.CurrentCulture);
            if (convOutput != null)
            {
                item = convOutput;
                bindedItemType = convOutput.GetType();
            }
        }
        return item;
    }

    private static object GetItem(object data, string[] elements, out Type itemType)
    {
        if (elements.Length == 0)
        {
            itemType = data.GetType();
            return data;
        }
        if (elements.Length == 1)
        {
            var accesor = data.GetType().GetProperty(elements[0]);
            itemType = accesor.PropertyType;
            return accesor.GetValue(data, null);
        }
        string[] innerElements = elements.Skip(1).ToArray();
        object innerData = data.GetType().GetProperty(elements[0]).GetValue(data);
        return GetItem(innerData, innerElements, out itemType);
    }