System.Windows.Controls.PopupHelper.Arrange C# (CSharp) Method

        public void Arrange()
        {
            if(Popup == null
                || PopupChild == null
#if SILVERLIGHT
 || OutsidePopupCanvas == null
#endif
 || Application.Current == null
#if SILVERLIGHT
 || Application.Current.Host == null
                || Application.Current.Host.Content == null
#endif
                // ReSharper disable RedundantLogicalConditionalExpressionOperand
 || false)
            // ReSharper restore RedundantLogicalConditionalExpressionOperand
            {
                return;
            }

#if SILVERLIGHT
            Content hostContent = Application.Current.Host.Content;
            double rootWidth = hostContent.ActualWidth;
            double rootHeight = hostContent.ActualHeight;
#else
            UIElement u = Parent;
            if(Application.Current.Windows.Count > 0)
            {
                // TODO: USE THE CURRENT WINDOW INSTEAD! WALK THE TREE!
                u = Application.Current.Windows[0];
            }
            while(u as Window == null && u != null)
            {
                u = VisualTreeHelper.GetParent(u) as UIElement;
            }
            Window w = u as Window;
            if(w == null)
            {
                return;
            }

            double rootWidth = w.ActualWidth;
            double rootHeight = w.ActualHeight;
#endif

            double popupContentWidth = PopupChild.ActualWidth;
            double popupContentHeight = PopupChild.ActualHeight;

            // ReSharper disable CompareOfFloatsByEqualityOperator
            if(rootHeight == 0 || rootWidth == 0 || popupContentWidth == 0 || popupContentHeight == 0)
            // ReSharper restore CompareOfFloatsByEqualityOperator
            {
                return;
            }

            const double rootOffsetX = 0;
            const double rootOffsetY = 0;

            double myControlHeight = Parent.ActualHeight;
            double myControlWidth = Parent.ActualWidth;

            // Use or come up with a maximum popup height.
            double popupMaxHeight = MaxDropDownHeight;
            if(double.IsInfinity(popupMaxHeight) || double.IsNaN(popupMaxHeight))
            {
                popupMaxHeight = (rootHeight - myControlHeight) * 3 / 5;
            }

            popupContentWidth = Math.Min(popupContentWidth, rootWidth);
            popupContentHeight = Math.Min(popupContentHeight, popupMaxHeight);
            popupContentWidth = Math.Max(myControlWidth, popupContentWidth);

            // We prefer to align the popup box with the left edge of the 
            // control, if it will fit.
            double popupX = rootOffsetX;
            if(rootWidth < popupX + popupContentWidth)
            {
                // Since it doesn't fit when strictly left aligned, we shift it 
                // to the left until it does fit.
                popupX = rootWidth - popupContentWidth;
                popupX = Math.Max(0, popupX);
            }

            // We prefer to put the popup below the combobox if it will fit.
            bool below = true;
            double popupY = rootOffsetY + myControlHeight;
            if(rootHeight < popupY + popupContentHeight)
            {
                below = false;
                // It doesn't fit below the combobox, lets try putting it above 
                // the combobox.
                popupY = rootOffsetY - popupContentHeight;
                if(popupY < 0)
                {
                    // doesn't really fit below either.  Now we just pick top 
                    // or bottom based on wich area is bigger.
                    if(rootOffsetY < (rootHeight - myControlHeight) / 2)
                    {
                        below = true;
                        popupY = rootOffsetY + myControlHeight;
                    }
                    else
                    {
                        popupY = rootOffsetY - popupContentHeight;
                    }
                }
            }

            // Now that we have positioned the popup we may need to truncate 
            // its size.
            popupMaxHeight = below ? Math.Min(rootHeight - popupY, popupMaxHeight) : Math.Min(rootOffsetY, popupMaxHeight);

            Popup.HorizontalOffset = 0;
            Popup.VerticalOffset = 0;

#if SILVERLIGHT
            OutsidePopupCanvas.Width = rootWidth;
            OutsidePopupCanvas.Height = rootHeight;

            // Transform the transparent canvas to the plugin's coordinate 
            // space origin.
            Matrix transformToRootMatrix = mt.Matrix;
            Matrix newMatrix;
            transformToRootMatrix.Invert(out newMatrix);
            mt.Matrix = newMatrix;

            OutsidePopupCanvas.RenderTransform = mt;
#endif
            PopupChild.MinWidth = myControlWidth;
            PopupChild.MaxWidth = rootWidth;
            PopupChild.MinHeight = 0;
            PopupChild.MaxHeight = Math.Max(0, popupMaxHeight);

            PopupChild.Width = popupContentWidth;
            PopupChild.HorizontalAlignment = HorizontalAlignment.Left;
            PopupChild.VerticalAlignment = VerticalAlignment.Top;

            // Set the top left corner for the actual drop down.
            Canvas.SetLeft(PopupChild, popupX - rootOffsetX);
            Canvas.SetTop(PopupChild, popupY - rootOffsetY);
        }