User Control position in WPF- Problems with PointToScreen

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Visual-Studio

XAML

WPF

C#5.0

Hi ,
I have a WPF app and I am using the library Modern UI too. I am trying to situate on a Page my user control in a relative position of the grid/screen. My final objective is to position my user controls (over a shape or background image) in x/y positions defined by me.



I have been reading about this topic and they recomend to use:

Point locationFromWindow = myusercontrol.TranslatePoint(new Point(0, 0),this );
Point locationFromScreen = myusercontrol.PointToScreen(locationFromWindow);

It looks simple but is giving me next error:

For now I am testing it with a simple button in my page:

<Grid Style="{StaticResource ContentRoot}">
      <Grid ShowGridLines="True" x:Name="mainGrid">
        <Grid.ColumnDefinitions>
          <ColumnDefinition Width="*" />
          <ColumnDefinition Width="*" />
          <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
          <Button   Content="Button" x:Name="b1"                            />

    </Grid>
    </Grid>

In the code behind of the page "Home" I added:

public partial class Home : UserControl
    {
        public Home()
        { 
            InitializeComponent();
            //Button b1 = new Button();
            Point locationFromWindow = b1.TranslatePoint(new Point(0, 0),this );
            Point locationFromScreen = b1.PointToScreen(locationFromWindow);
           // mainLayout.Children.Add(b1);

        }
    }

It returns a parsing error: '

Additional information: 'The invocation of the constructor on type Test.Pages.Home' that matches the specified binding constraints threw an exception.' Line number '8' and line position '6'.

I tried adding the button from the code behind too ( and not from the XAML) with the commented lines , but it also didn't works.

Why is this happening? Any suggestion for a better performance of my objective?