C# WPF how to draw a circle which extends to fill its parent and remains a circle in code behind

Question

I have to draw a circle in a grid. That grid has to adapt proportionally to height and width defined by the Column/Row definition of its parent grid. Now if I put stretch it will fill all the space and become an ellipsis while I want it to be a circle.

Ok in short the parent grid adapts proportionally like that

then in a routine I add the following code:

public void RadialPercentage(Grid grd )
{
    Ellipse elpExt = new Ellipse();
    elpExt.Stroke = Brushes.Green;
    elpExt.StrokeThickness = 4;
    //elpExt.Margin = new Thickness(0);
    //elpExt.HorizontalAlignment = HorizontalAlignment.Center;
    elpExt.VerticalAlignment = VerticalAlignment.Stretch;
    grd.Children.Add(elpExt);

    Ellipse elpInt = new Ellipse();
    elpInt.Stroke = Brushes.Blue;
    elpInt.StrokeThickness = 4;
    elpInt.Margin = new Thickness(20);
    //elpInt.Width = elpInt.Height = dim-20;
    //elpInt.HorizontalAlignment = HorizontalAlignment.Center;
    elpInt.VerticalAlignment = VerticalAlignment.Stretch;
    grd.Children.Add(elpInt);
    return;
}

but the effect is the following:

so it stretches both vertically and horizontally even if I only put the vertical and not the horizontal constraint. If I set it to center the ellipse collapses.

To solve the problem even I am not sure that this is the right thing to do I tried to take a look of the weight/heigth of the parent grid but obviously both those values and the actual values are set to zero.

thanks for helping Patrick

Answer 1

What about setting Width 's binding to ActualHeight of the ellipse and set HorizontalAlignment to Center ? Something like this:

var ellipse = new Ellipse();

var binding = new Binding(Ellipse.ActualHeightProperty.Name)
{
    RelativeSource = new RelativeSource(RelativeSourceMode.Self),
    Mode = BindingMode.OneWay
};

ellipse.VerticalAlignment = VerticalAlignment.Stretch;
ellipse.HorizontalAlignment = HorizontalAlignment.Center;

BindingOperations.SetBinding(ellipse, Ellipse.WidthProperty, binding);

Answer 2

You can update the size of your Ellipse each time the parent Grid is resized.

You should add to your Grid the SizeChanged Event . XAML example:

<Grid Name        = "MyGrid"
      SizeChanged = "MyGridSizeChanged">
  <!-- rows and columns definitions -->
  <Ellipse Name        = "MyEllipse"
           Grid.Row    = "i"
           Grid.Column = "j" />
</Grid>

Now, each time the Grid is resized the function MyGridSizeChanged will executed. You should add into it code, which set sizes of your Ellipse equal to smallest side of contained cell. C# example:

void MyGridSizeChanged(object sender, SizeChangedEventArgs e) {
    if (sender is Grid myGrid) {
        var cellHeight = myGrid.RowDefinitions[Grid.GetRow(MyEllipse)].ActualHeight;
        var cellWidth  = myGrid.ColumnDefinitions[Grid.GetColumn(MyEllipse)].ActualWidth;
        var newSize    = Math.Min(cellHeight, cellWidth);

        MyEllipse.Height = newSize;
        MyEllipse.Width  = newSize;
    }
}