How to create a control inside a border with a corner radius

Question

I have a simple window containing an outer border with an angular radius and an inner border with a background. The border is basically just a placeholder for any type of content that I would like to place inside a rounded outer border.

<Window x:Class="TestRunner.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525" AllowsTransparency="True" 
        WindowStyle="None" Background="{x:Null}" >
    <Border BorderThickness="2" BorderBrush="Black" CornerRadius="8"  >     
           <Border Background="White">

           </Border>
    </Border>
</Window>

The problem is that the internal control does not inherit the rounded corner, so it draws a rounded corner on top, for example:

How to set up an external control, so the internal controls do not try to draw over a rounded corner.

Setting a rounded corner on an internal control is not a viable option, as this will lead to a terrible doubling of the corner radius.

Answer 1

, Border Border, . , , Border, - .

, - , Clip:

<Border x:Name="border" CornerRadius="10">
    <Border.Clip>
        <RectangleGeometry Width="{Binding ActualWidth, ElementName=border}" Height="{Binding ActualHeight, ElementName=border}" RadiusX="10" RadiusY="10"/>
    </Border.Clip>

    <Border Background="White"/>
</Border>

( ) Border "" . Fill IsHitTestVisible, false, :

<Grid>
    <Border Background="White"/>
    <Border CornerRadius="10" BorderBrush="Black" BorderThickness="3" Fill="Transparent" IsHitTestVisible="false"/>
</Grid>

Answer 2

RectangleGeometry Width, WPF.

IMultiValueConverter, : fooobar.com/questions/134279/...

, ( , Border.Fill ).

:

<Grid>
    <Border x:Name="canvasBorder" CornerRadius="5">
        <Border.Resources>
            <tools:ContentClipConverter x:Key="ContentClipConverter" />
        </Border.Resources>
        <Border.Clip>
            <MultiBinding Converter="{StaticResource ContentClipConverter}">
                <Binding Path="ActualWidth"
                RelativeSource="{RelativeSource Self}"/>
                <Binding Path="ActualHeight"
                RelativeSource="{RelativeSource Self}"/>
                <Binding Path="CornerRadius"
                RelativeSource="{RelativeSource Self}"/>
            </MultiBinding>
        </Border.Clip>
        <!-- ... -->
    </Border>
    <Border BorderBrush="Black" BorderThickness="1" CornerRadius="5" Background="Transparent" IsHitTestVisible="false" />
</Grid>

ContentClipConverter.cs:

/// <summary>
/// Clips the content of a rounded corner border.
/// Code taken from <a href="/questions/134279/how-to-make-the-border-trim-the-child-elements/803232#803232">this topic</a>
/// </summary>
public class ContentClipConverter : IMultiValueConverter {
    /// <summary>
    /// Gets a clipping geometry for the item
    /// </summary>
    /// <param name="values">The input values</param>
    /// <param name="targetType">The parameter is not used.</param>
    /// <param name="parameter">The parameter is not used.</param>
    /// <param name="culture">The parameter is not used.</param>
    /// <returns>The clipping geometry</returns>
    public object Convert(object[] values, Type targetType, object parameter, CultureInfo culture) {
        if (values.Length == 3 && values[0] is double && values[1] is double && values[2] is CornerRadius) {
            var width = (double)values[0];
            var height = (double)values[1];

            if (width < double.Epsilon || height < double.Epsilon) {
                return Geometry.Empty;
            }

            var radius = (CornerRadius)values[2];

            // Actually we need more complex geometry, when CornerRadius has different values.
            // But let me not to take this into account, and simplify example for a common value.
            var clip = new RectangleGeometry(new Rect(0, 0, width, height), radius.TopLeft, radius.TopLeft);
            clip.Freeze();

            return clip;
        }

        return DependencyProperty.UnsetValue;
    }

    /// <summary>
    /// Not implemented
    /// </summary>
    /// <param name="value">The parameter is not used.</param>
    /// <param name="targetTypes">The parameter is not used.</param>
    /// <param name="parameter">The parameter is not used.</param>
    /// <param name="culture">The parameter is not used.</param>
    /// <returns>This function does not return anything</returns>
    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, CultureInfo culture) {
        throw new NotSupportedException();
    }

Answer 3

One possibility might be to draw an outer border:

<Border BorderThickness="2" BorderBrush="Black" CornerRadius="8" Padding="4">
    ....
</Border>

but this can lead to too much space in your application.