Dependency property callback on datagrid custom column not firing

Question

I have the following custom datagrid column, for passing in the value of the DatePart dependency property as the ConverterParameter to the editing element's converter:

Public Class DataGridTimeColumn
    Inherits DataGridTextColumn

    Shared ReadOnly DatePartProperty = DependencyProperty.Register("DatePart", GetType(DateTime?), GetType(DataGridTimeColumn), New FrameworkPropertyMetadata(Nothing, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, AddressOf RefreshBinding))
    Property DatePart As DateTime?
        Get
            Return GetValue(DatePartProperty)
        End Get
        Set(value As DateTime?)
            SetValue(DatePartProperty, value)
        End Set
    End Property

    Private Shared Sub RefreshBinding(d As DependencyObject, e As DependencyPropertyChangedEventArgs)
        Dim tc As DataGridTimeColumn = d
        tc.Binding = tc.Binding
    End Sub

    Public Overrides Property Binding As BindingBase
        Get
            Return MyBase.Binding
        End Get
        Set(value As BindingBase)
            Dim b As Data.Binding = value
            With b
                .Converter = New TimeConverter
                .ConverterParameter = DatePart
            End With
            MyBase.Binding = b
        End Set
    End Property
End Class

With the following XAML:

<my:DataGridTimeColumn Header="From" Binding="{Binding FromDate}" DatePart="{Binding FromDate}" />
<my:DataGridTimeColumn Header="Until" Binding="{Binding TillDate}" DatePart="{Binding TillDate}" />

But RefreshBinding is never called (I've set a breakpoint and it's never triggered), and thus DatePart is always Nothing(null) when the ConverterParameter is set. How can I fix this?

Edit

In C#:

public class DataGridTimeColumn : DataGridTextColumn
{
    static readonly DependencyProperty DatePartProperty = DependencyProperty.Register(
        "DatePart", typeof(DateTime?), typeof(DataGridTimeColumn),
        new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, RefreshBinding)
    );

    public DateTime? DatePart
    {
        get { return (DateTime?)GetValue(DatePartProperty); }
        set { SetValue(DatePartProperty, value); }
    }

    private static void RefreshBinding(DependencyObject d, DependencyPropertyChangedEventArgs e) 
    {
        var tc = (DataGridTimeColumn)d;
        tc.Binding = tc.Binding;
    }

    public override System.Windows.Data.BindingBase Binding
    {
        get { return base.Binding; }
        set
        { 
            var b = (Binding)value;
            b.Converter = new TimeConverter();
            b.ConverterParameter = DatePart;
            base.Binding = b;
        }
    }
}

Answer 1

I might not understand what you're exactly trying to do, but this seems like a lot of work to just view/edit the DatePart. Would using a formatter and a DataGridTextColumn be easier? WPF Binding StringFormat Short Date String

Answer 2

First, please use debug to check your binding path whether it is ok. Second, check your FromDate property whether is triggered. Third, please use Snoop to monitor this DP whether is updated.

Last, i remember DataGrid has dynamic binding bug, so i am not sure above steps can solve your problem. If it is not ok , please let me know.

Answer 3

I would recommend Darlene approach if you only need to format the value. If you really need a converter for some magic I would recommend to set it via your binding in the XAML (I don't really know why you are trying to set it in the property definition).

Your XAML will be something like this:

 <my:DataGridTimeColumn Header="From" Binding="{Binding FromDate, Converter={StaticResource myTimeConverter}} />

TimeConverter must implement IValueConverter and an instance must be defined in your window/datagrid resources. You can also define the TwoWay in that binding.

You can find a full example here.

In case you want to combine two or more pieces of data you can use a MultiBinding.

Answer 4

Tried to understand what you want to achieve from the comments and if I got it right you can do it using only the view model and a converter with a small workaround:

In your converter ConvertBack method, you can try to figure out if the value entered by the user has only the time part and if so set the date to something that will not be used (for example DateTime.MinValue):

public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
    var convertedDateTime = DateTime.Parse(value.ToString());
    var hasOnlyTime = value.ToString().Trim().Length <= 5; // This should be replaced with something more consistent

    if (hasOnlyTime)
        return new DateTime(DateTime.MinValue.Year, DateTime.MinValue.Month, DateTime.MinValue.Day, convertedDateTime.Hour, convertedDateTime.Minute, convertedDateTime.Second);

    return convertedDateTime;
}

Then in the object setter for the date you can check that and use the required value, if the date part is DateTime.MinValue change it back to the previous one, if not leave it as it is:

 private DateTime _fromDate;
 public DateTime FromDate
 {
    get { return _fromDate; }
    set 
    {
        if (value.Date != DateTime.MinValue)
            _fromDate = value;
        else
            _fromDate = new DateTime(_fromDate.Year, _fromDate.Month, _fromDate.Day, value.Hour, value.Minute, value.Second);

        OnPropertyChanged("FromDate");
    }
 }

Tested on .NET 4.5 and works as expected.