Question

I try to rotate a Border and have the MainWindow change his size based on the new space taken by the Border rotation.
I’ve set SizeToContent="WidthAndHeight" but window size does’t take change when I rotated the border.

Do I need to programmatically set the Width and Height for the MainWindow or this can be achieved changing the xaml code in some other way?

My xaml code:

<Window x:Class="MyClass.MainWindow"
        WindowStyle="None" AllowsTransparency='True' 
        Topmost='False' Background="Transparent"  ShowInTaskbar='False' 
        SizeToContent="WidthAndHeight" WindowStartupLocation="Manual">
    <Border Name="MyBorder" 
            BorderBrush="Transparent"
            Background="Transparent"
            HorizontalAlignment="Left" 
            VerticalAlignment="Top"
            RenderTransformOrigin="0.5,0.5">
    </Border>
</Windows>

My c# code on Window_KeyDown:

# RotateTransform rt = new RotateTransform() is declared at class level.

if (e.Key == Key.I)
                {
                    if (rt.Angle + 1 < 360)
                    {
                        rt.Angle += 1;                    
                    }
                    else
                    {
                        rt.Angle = 0;                   
                    }    



                MyBorder.RenderTransform = rt;
            }

Answer 1

Use LayoutTransform instead of RenderTransform

From MSDN: Transforms Overview

• LayoutTransform – A transform that is applied before the layout pass. After the transform is applied, the layout system processes the
  transformed size and position of the element.

• RenderTransform – A transform that modifies the appearance of the element but is applied after the layout pass is complete. By using the
  RenderTransform property instead of the LayoutTransform property, you
  can obtain performance benefits.

Example

<Border Name="MyBorder"  
        BorderBrush="Transparent" 
        Background="Transparent" 
        HorizontalAlignment="Left"  
        VerticalAlignment="Top" 
        RenderTransformOrigin="0.5,0.5">
    <Border.LayoutTransform>
        <RotateTransform Angle="90"/>
    </Border.LayoutTransform>
</Border>

So in your case

RotateTransform rt = new RotateTransform(0.0, 0.5, 0.5);
private void Window_KeyDown(object sender, KeyEventArgs e)
{
    if (e.Key == Key.I)
    {
        if (rt.Angle + 1 < 360)
        {
            rt.Angle += 1;
        }
        else
        {
            rt.Angle = 0;
        }
        MyBorder.LayoutTransform = rt;
    }
}}