Question

I need to create a line with the first point centered to the Grid it is contained in (without manually setting Width/Height of Grid). Here is very simple code sample I want to update so I have one point of “Line1” centered within “Grid1”.

<Window
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    x:Class="SimpleLine.MainWindow"
    x:Name="Window"
    Title="MainWindow"
    Width="640" Height="480">
    <Grid x:Name="LayoutRoot">
        <Grid Name="Grid1">
            <Line Name="Line1" X2="200" Y2="100" Stroke="Black"></Line>
        </Grid>
    </Grid>
</Window>

When I try in code behind:

Line1.X1 = Grid1.Widht/2; Line1.Y1 = Grid1.Height/2;

I get an error (uncaught exception) – that “Not a number” is not a valid value for X1(Y1).

Thanks for any efforts.

PS: I am rather a WPF-beginner.

Answer 1

In code behind you will have to use ActualWidth and ActualHeight if you have not explicitly set the Width/Height.

Line1.X1 = Grid1.ActualWidth / 2; Line1.Y1 = Grid1.ActualHeight / 2;

if you want to do in XAML you can make BindingConverters to perform logic on a binded value.

Xaml:

<Window x:Class="WpfApplication6.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:local="clr-namespace:WpfApplication6"
        Title="MainWindow" Height="350" Width="525">
    <Window.Resources>
        <local:DivideByConverter x:Key="Divider" />
    </Window.Resources>
    <Grid x:Name="LayoutRoot">
        <Line Name="Line1" X2="{Binding ElementName=LayoutRoot, Path=ActualWidth, Converter={StaticResource ResourceKey=Divider}}" Y2="{Binding ElementName=LayoutRoot, Path=ActualHeight, Converter={StaticResource ResourceKey=Divider}}" Stroke="Black"/>
    </Grid>
</Window>

Binding converter:

 public class DivideByConverter : IValueConverter
    {
        /// <summary>
        /// Converts a value.
        /// </summary>
        /// <param name="value">The value produced by the binding source.</param>
        /// <param name="targetType">The type of the binding target property.</param>
        /// <param name="parameter">The xmlentry to the language value</param>
        /// <param name="culture">The culture to use in the converter.</param>
        /// <returns>
        /// A converted value. If the method returns null, the valid null value is used.
        /// </returns>
        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            int divider = 2;
            if (value is double)
            {
                return (double)value / divider;
            }
            return value;
        }

        /// <summary>
        /// Converts a value.
        /// </summary>
        /// <param name="value">The value that is produced by the binding target.</param>
        /// <param name="targetType">The type to convert to.</param>
        /// <param name="parameter">The converter parameter to use.</param>
        /// <param name="culture">The culture to use in the converter.</param>
        /// <returns>
        /// A converted value. If the method returns null, the valid null value is used.
        /// </returns>
        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            throw new Exception("The method or operation is not implemented.");
        }
    }

This will take the Binded value (ActualWidth) and divide it by two and the same for ActualHeight.
So even if your from is resized the line will stay in the center