WPF Binding and Dynamically Assigning StringFormat Property

bindingdynamicstring-formattingwpf

I have a form that is generated based on several DataTemplate elements. One of the DataTemplate elements creates a TextBox out of a class that looks like this:

public class MyTextBoxClass
{
   public object Value { get;set;}
   //other properties left out for brevity's sake
   public string FormatString { get;set;}
}

I need a way to "bind" the value in the FormatString property to the "StringFormat" property of the binding. So far I have:

<DataTemplate DataType="{x:Type vm:MyTextBoxClass}">
 <TextBox Text="{Binding Path=Value, StringFormat={Binding Path=FormatString}" />
</DataTemplate>

However, since StringFormat isn't a dependency property, I cannot bind to it.

My next thought was to create a value converter and pass the FormatString property's value in on the ConverterParameter, but I ran into the same problem — ConverterParameter isn't a DependencyProperty.

So, now I turn to you, SO. How do I dynamically set the StringFormat of a binding; more specifically, on a TextBox?

I would prefer to let XAML do the work for me so I can avoid playing with code-behind. I'm using the MVVM pattern and would like to keep the boundaries between view-model and view as un-blurred as possible.

Thanks!

Best Answer

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;
    }
}

Related Solutions

C# – Binding a WPF ComboBox to a custom list

You set the DisplayMemberPath and the SelectedValuePath to "Name", so I assume that you have a class PhoneBookEntry with a public property Name.

Have you set the DataContext to your ConnectionViewModel object?

I copied you code and made some minor modifications, and it seems to work fine. I can set the viewmodels PhoneBookEnty property and the selected item in the combobox changes, and I can change the selected item in the combobox and the view models PhoneBookEntry property is set correctly.

Here is my XAML content:

<Window x:Class="WpfApplication6.Window1"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Window1" Height="300" Width="300">
<Grid>
    <StackPanel>
        <Button Click="Button_Click">asdf</Button>
        <ComboBox ItemsSource="{Binding Path=PhonebookEntries}"
                  DisplayMemberPath="Name"
                  SelectedValuePath="Name"
                  SelectedValue="{Binding Path=PhonebookEntry}" />
    </StackPanel>
</Grid>
</Window>

And here is my code-behind:

namespace WpfApplication6
{

    /// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();
            ConnectionViewModel vm = new ConnectionViewModel();
            DataContext = vm;
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            ((ConnectionViewModel)DataContext).PhonebookEntry = "test";
        }
    }

    public class PhoneBookEntry
    {
        public string Name { get; set; }

        public PhoneBookEntry(string name)
        {
            Name = name;
        }

        public override string ToString()
        {
            return Name;
        }
    }

    public class ConnectionViewModel : INotifyPropertyChanged
    {
        public ConnectionViewModel()
        {
            IList<PhoneBookEntry> list = new List<PhoneBookEntry>();
            list.Add(new PhoneBookEntry("test"));
            list.Add(new PhoneBookEntry("test2"));
            _phonebookEntries = new CollectionView(list);
        }

        private readonly CollectionView _phonebookEntries;
        private string _phonebookEntry;

        public CollectionView PhonebookEntries
        {
            get { return _phonebookEntries; }
        }

        public string PhonebookEntry
        {
            get { return _phonebookEntry; }
            set
            {
                if (_phonebookEntry == value) return;
                _phonebookEntry = value;
                OnPropertyChanged("PhonebookEntry");
            }
        }

        private void OnPropertyChanged(string propertyName)
        {
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
        public event PropertyChangedEventHandler PropertyChanged;
    }
}

Edit: Geoffs second example does not seem to work, which seems a bit odd to me. If I change the PhonebookEntries property on the ConnectionViewModel to be of type ReadOnlyCollection, the TwoWay binding of the SelectedValue property on the combobox works fine.

Maybe there is an issue with the CollectionView? I noticed a warning in the output console:

System.Windows.Data Warning: 50 : Using CollectionView directly is not fully supported. The basic features work, although with some inefficiencies, but advanced features may encounter known bugs. Consider using a derived class to avoid these problems.

Edit2 (.NET 4.5): The content of the DropDownList can be based on ToString() and not of DisplayMemberPath, while DisplayMemberPath specifies the member for the selected and displayed item only.

WPF: Textbox Binding with StringFormat={}{0:F2}. Don’t show zero’s

Try the following:

StringFormat={}{0:#.##}

It will format to two decimal places and won't show zeroes.

Best Answer

Related Solutions

C# – Binding a WPF ComboBox to a custom list

WPF: Textbox Binding with StringFormat={}{0:F2}. Don’t show zero’s

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