Question

Let’s say I have a Border whose DataContext is an object of type MyViewModel. MyViewModel has bool properties called RoundLeft and RoundRight. When RoundLeft is true, I want the CornerRadius of the border to be 6,0,0,6. When RoundRight is true, I want 0,6,6,0. When both are true, I want 6,6,6,6.

I’ve described my first two attempts below. I haven’t given up yet, but I wanted to see if anyone else might have any ideas.

Attempt #1

I got it partially working by binding to the MyViewModel instance itself (not a specific property) and using an IValueConverter that builds the correct CornerRadius object. This works on initial load. The problem is that the binding is monitoring changes of the object as a whole rather than changes to the specific RoundLeft and RoundRight properties, e.g. if RoundLeft changes, the border’s CornerRadius doesn’t.

Binding:

<Border CornerRadius="{Binding Converter={StaticResource myShiftCornerRadiusConverter}}" />

Converter:

public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        var myViewModel = value as MyViewModel;
        if (myViewModel != null)
        {
            return new CornerRadius(
                myViewModel.RoundLeft ? 6 : 0,
                myViewModel.RoundRight ? 6 : 0,
                myViewModel.RoundRight ? 6 : 0,
                myViewModel.RoundLeft ? 6 : 0);
        }
        else
        {
            return new CornerRadius(6);
        }
    }

Attempt #2

This blog post from Colin Eberhardt looked promising, but I’m getting vague XamlParseExceptions and ComExceptions. Here’s my XAML:

<Border>
<ce:MultiBindings>
    <ce:MultiBinding TargetProperty="CornerRadius" Converter="{StaticResource myCornerRadiusConverter}">
        <ce:MultiBinding.Bindings>
            <ce:BindingCollection>
                <Binding Path="RoundLeft" />
                <Binding Path="RoundRight" />
            </ce:BindingCollection>
        </ce:MultiBinding.Bindings>
    </ce:MultiBinding>
</ce:MultiBindings>
</Border>

Here’s my converter, although the execution never gets this far, i.e. my breakpoint is never hit.

public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        if (values.Length == 2 && values.All(v => v is bool))
        {
            var roundLeft = (bool)values[0];
            var roundRight = (bool)values[1];

            return new CornerRadius(
                roundLeft ? 6 : 0,
                roundRight ? 6 : 0,
                roundRight ? 6 : 0,
                roundLeft ? 6 : 0);
        }
        else
        {
            return new CornerRadius(6);
        }
    }

Answer 1

I implemented the approach @Foovanadil suggested, but then I got another idea: I created a new ContentControl that exposes RoundLeft and RoundRight dependency properties. It certainly involved more code, but now the CornerRadius stuff is all in the View layer.

[TemplatePart(Name = _borderPartName, Type = typeof(Border))]
public class CustomRoundedBorder : ContentControl
{
    #region Private Fields

    private const string _borderPartName = "PART_Border";
    private Border _borderPart;

    #endregion

    #region Dependency Properties

    #region DefaultCornerRadius

    //////////////////////////////////////////////////////////////////////////////
    /// <summary>
    /// Gets or sets the default corner radius, in pixels.
    /// </summary>
    //////////////////////////////////////////////////////////////////////////////
    public double DefaultCornerRadius
    {
        get { return (double)GetValue(DefaultCornerRadiusProperty); }
        set { SetValue(DefaultCornerRadiusProperty, value); }
    }

    public static readonly DependencyProperty DefaultCornerRadiusProperty = DependencyProperty.Register(
        "DefaultCornerRadius", typeof(double), typeof(CustomRoundedBorder),
        new PropertyMetadata(new PropertyChangedCallback(RoundingChanged)));

    #endregion

    #region RoundLeft

    //////////////////////////////////////////////////////////////////////////////
    /// <summary>
    /// Gets or sets a value indicating whether to round the corners on the left side of the border.
    /// </summary>
    //////////////////////////////////////////////////////////////////////////////
    public bool RoundLeft
    {
        get { return (bool)GetValue(RoundLeftProperty); }
        set { SetValue(RoundLeftProperty, value); }
    }

    public static readonly DependencyProperty RoundLeftProperty = DependencyProperty.Register(
        "RoundLeft", typeof(bool), typeof(CustomRoundedBorder),
        new PropertyMetadata(new PropertyChangedCallback(RoundingChanged)));

    #endregion

    #region RoundRight

    //////////////////////////////////////////////////////////////////////////////
    /// <summary>
    /// Gets or sets a value indicating whether to round the corners on the left side of the border.
    /// </summary>
    //////////////////////////////////////////////////////////////////////////////
    public bool RoundRight
    {
        get { return (bool)GetValue(RoundRightProperty); }
        set { SetValue(RoundRightProperty, value); }
    }

    public static readonly DependencyProperty RoundRightProperty = DependencyProperty.Register(
        "RoundRight", typeof(bool), typeof(CustomRoundedBorder),
        new PropertyMetadata(new PropertyChangedCallback(RoundingChanged)));

    #endregion

    #region EffectiveCornerRadius

    //////////////////////////////////////////////////////////////////////////////
    /// <summary>
    /// Gets the effective corner radius, based on DefaultCornerRadius and 
    /// RoundLeft and RoundRight.
    /// </summary>
    //////////////////////////////////////////////////////////////////////////////
    public double EffectiveCornerRadius
    {
        get { return (double)GetValue(EffectiveCornerRadiusProperty); }
        private set { SetValue(EffectiveCornerRadiusProperty, value); }
    }

    public static readonly DependencyProperty EffectiveCornerRadiusProperty = DependencyProperty.Register(
        "EffectiveCornerRadius", typeof(double), typeof(CustomRoundedBorder),
        new PropertyMetadata(new PropertyChangedCallback(RoundingChanged)));

    #endregion

    #endregion

    #region Overrides

    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();

        this._borderPart = this.GetTemplateChild(_borderPartName) as Border;
        this.UpdateCornerRadius();
    }

    #endregion

    #region Private Methods

    private static void RoundingChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var control = d as CustomRoundedBorder;
        if (control != null)
        {
            control.UpdateCornerRadius();
        }
    }

    private void UpdateCornerRadius()
    {
        if (this._borderPart != null)
        {
            this._borderPart.CornerRadius = new CornerRadius(
                this.RoundLeft ? this.DefaultCornerRadius : 0,
                this.RoundRight ? this.DefaultCornerRadius : 0,
                this.RoundRight ? this.DefaultCornerRadius : 0,
                this.RoundLeft ? this.DefaultCornerRadius : 0);
        }
    }

    #endregion
}

Then I created a ControlTemplate for it (some properties omitted for brevity):

<ControlTemplate x:Key="MyBorderTemplate" TargetType="ce:CustomRoundedBorder">
    <Border
        x:Name="PART_Border"
        CornerRadius="{TemplateBinding EffectiveCornerRadius}"
        >
        <ContentPresenter />
    </Border>
</ControlTemplate>

Then here’s where I bound it to the view-model properties:

<ce:CustomRoundedBorder
    DefaultCornerRadius="6"
    RoundLeft="{Binding RoundLeft}"
    RoundRight="{Binding RoundRight}"
    Template="{StaticResource MyBorderTemplate}"
    >
    <!-- Content -->
</ce:CustomRoundedBorder>