ActualWidth and ActualWidth is 0 - xaml

Question

First of all YES I've read all the suggested "Questions that may already have your answer", so I belive this is not a dublicate.
I have this grid on a Page:
<Grid>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<RowDefinition Height="*"/>
<RowDefinition Height="70"/>
</Grid.RowDefinitions>
<!--TitlePanel contains the name of the application and page title-->
<StackPanel Grid.Row="0" Margin="12,17,0,28">
<TextBlock Text="Please sign below." Margin="9,-7,0,0" Style="{StaticResource PhoneTextTitle1Style}"/>
</StackPanel>
<!--ContentPanel - place additional content here-->
<Grid x:Name="ContentPanel" Grid.Row="1" Margin="12,0,12,0">
<Image x:Name="SignImage" />
</Grid>
<Grid Background="Transparent" Grid.Row="2" Margin="12,0,10,0">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="120"/>
<ColumnDefinition Width="120"/>
<ColumnDefinition Width="10"/>
</Grid.ColumnDefinitions>
<Button x:Name="ClearButton" Content="Clear" Grid.Column="1"/>
<Button x:Name="OkButton" Content="Ok" Grid.Column="2"/>
</Grid>
</Grid>
Then this CodeBehind:
public MainPage()
{
InitializeComponent();
this.Loaded += MainPage_Loaded;
// Sample code to localize the ApplicationBar
//BuildLocalizedApplicationBar();
}
....
private void MainPage_Loaded(object sender, RoutedEventArgs e)
{
MessageBox.Show($"SignImage.ActualWidth: {SignImage.ActualWidth}, SignImage.ActualWidth: {SignImage.ActualWidth} ");
}
The problem is that the size of my Image is 0 (zero).
How do I make my Image fill out the cell I've placed it in?

Answer 1

First there is difference between * and Auto. Please do check the size of the main grid. "*" means stretch to fill the available space, and is a fraction, so if one row is * and the other row is 2* then the second row will be twice the size of the first row.
Auto means it takes whatever/or only space it needs.
If you have set the size of whole Grid to a small height then there will be a problem. Set the MinHeight for the Main Grid component.
Second you have not set the image for SignImage with a size which mean you will obviously get ActualWidth and ActualHeight as Zero. The image tag is empty in which no Source is set.
<Image x:Name="SignImage" Source="C:\Users\Administrator\Pictures\user-thumbnail.png"/>
Check after setting the image source if you are getting the ActualWidth and ActualHeight
Have a look at this image I have written your code. You can see that for Grid 0 and 2 there is Height set for Grid 1 there is no Height set as you have written * for it so it is necessary to set the height to parent.
Now look at this image I have set the height for the parent element as 200 and hence you can see the Gird 1 is visible and the height is not 0.
So it is necessary to set the height or set the image source for the height not to be zero.