Question

I’m building a WPF application in which I have a need to get the width, height and locations of the window from my view model. I’m using the following XAML:

<Window x:Class="ScreenCapture.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d"
        Height="{Binding Height, Mode=OneWayToSource, FallbackValue=300}"
        Width="{Binding Width, Mode=OneWayToSource, FallbackValue=300}"
        Title="MVVM Light Application"
        Top="{Binding Top, Mode=OneWayToSource}"
        Left="{Binding Left, Mode=OneWayToSource}"
        DataContext="{Binding Main, Source={StaticResource Locator}}">
    <Window.Resources>
        <ResourceDictionary>
            <ResourceDictionary.MergedDictionaries>
                <ResourceDictionary Source="Skins/MainSkin.xaml" />
            </ResourceDictionary.MergedDictionaries>
        </ResourceDictionary>
    </Window.Resources>

    <Grid x:Name="LayoutRoot">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="13*" />
            <ColumnDefinition Width="265*" />
        </Grid.ColumnDefinitions>
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition Height="25" />
        </Grid.RowDefinitions>
        <TextBlock FontSize="36"
                   FontWeight="Bold"
                   Foreground="Purple"
                   Text="{Binding Welcome}"
                   VerticalAlignment="Center"
                   HorizontalAlignment="Center"
                   TextWrapping="Wrap" Grid.Column="1" Margin="19,70,32,70" />
        <Button Grid.Row="1" Content="Capture" Grid.ColumnSpan="2" Command="{Binding Capture}" />
    </Grid>
</Window>

In my view model, the values for the Top, Left work fine, but, the Width and Height properties are both NaN. I have properties like this for each of the attributes I’m binding on:

private double height;

public double Height
{
    set
    {
        height = value;
    }
}

Any idea why the values are coming back NaN? More importantly, what can I do to get the values I’m looking for?

EDIT: I realize this is a bit off, but I need to capture a screenshot of the application from the view model. The view model contains additional properties that I didn’t paste, but are necessary in order to implement the desired functionality. For the curious, below is my method for capturing the screen:

private void CaptureScreen()
{
    int x = Convert.ToInt32(left);
    int y = Convert.ToInt32(top);
    int w = Convert.ToInt32(width);
    int h = Convert.ToInt32(height);
    Bitmap image = new Bitmap(w, h);
    Graphics graphics = Graphics.FromImage(image);

    graphics.CopyFromScreen(x, y, x, y, new Size(w, h), CopyPixelOperation.SourceCopy);

    // Do something with the image
}

Answer 1

As mzabsky said, you need to use ActualWidth and ActualHeight. The problem here is that they are readonly and you can’t do a OneWayToSource Binding on a readonly Dependency Property.

However, there are workarounds.
See the following question: Pushing read-only GUI properties back into ViewModel

The attached behavior provided by Kent Boogaart lets you do this

<Window ...
        SizeObserver.Observe="True"
        SizeObserver.ObservedWidth="{Binding Width, Mode=OneWayToSource}"
        SizeObserver.ObservedHeight="{Binding Height, Mode=OneWayToSource}">

or you can try the solution I made for this called PushBinding which can be used to push readonly DP’s to the viewmodel. In your case the usage would look like this

<Window ...>
    <pb:PushBindingManager.PushBindings> 
        <pb:PushBinding TargetProperty="ActualHeight" Path="Height"/> 
        <pb:PushBinding TargetProperty="ActualWidth" Path="Width"/>
    </pb:PushBindingManager.PushBindings> 
</Window> 

You can download a demo project using PushBinding here if you’re interested: https://www.dropbox.com/s/eqkmsp0q7hb568z/PushBindingInStyleDemo.zip?dl=0