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Layout Considerations for the WindowsFormsHost Element

This topic describes how the WindowsFormsHost element interacts with the WPF layout system.

WPF and Windows Forms support different, but similar, logic for sizing and positioning elements on a form or page. When you create a hybrid user interface (UI) that hosts Windows Forms controls in WPF, the WindowsFormsHost element integrates the two layout schemes.

WPF uses resolution-independent layout. All WPF layout dimensions are specified using device-independent pixels. A device-independent pixel is one ninety-sixth of an inch in size and resolution-independent, so you get similar results regardless of whether you are rendering to a 72-dpi monitor or a 19,200-dpi printer.

WPF is also based on dynamic layout. This means that a UI element arranges itself on a form or page according to its content, its parent layout container, and the available screen size. Dynamic layout facilitates localization by automatically adjusting the size and position of UI elements when the strings they contain change length.

Layout in Windows Forms is device-dependent and more likely to be static. Typically, Windows Forms controls are positioned absolutely on a form using dimensions specified in hardware pixels. However, Windows Forms does support some dynamic layout features, as summarized in the following table.

Layout feature

Description

Autosizing

Some Windows Forms controls resize themselves to display their contents properly. For more information, see AutoSize Property Overview.

Anchoring and docking

Windows Forms controls support positioning and sizing based on the parent container. For more information, see Control.Anchor and Control.Dock.

Autoscaling

Container controls resize themselves and their children based on the resolution of the output device or the size, in pixels, of the default container font. For more information, see Automatic Scaling in Windows Forms.

Layout containers

The FlowLayoutPanel and TableLayoutPanel controls arrange their child controls and size themselves according to their contents.

In general, Windows Forms controls cannot be scaled and transformed to the extent possible in WPF. The following list describes the known limitations when the WindowsFormsHost element attempts to integrate its hosted Windows Forms control into the WPF layout system.

  • In some cases, Windows Forms controls cannot be resized, or can be sized only to specific dimensions. For example, a Windows Forms ComboBox control supports only a single height, which is defined by the control's font size. In a WPF dynamic layout where elements can stretch vertically, a hosted ComboBox control will not stretch as expected.

  • Windows Forms controls cannot be rotated or skewed. The WindowsFormsHost element raises the LayoutError event if you apply a skew or rotation transformation. If you do not handle the LayoutError event, an InvalidOperationException is raised.

  • In most cases, Windows Forms controls do not support proportional scaling. Although the overall dimensions of the control will scale, child controls and component elements of the control may not resize as expected. This limitation depends on how well each Windows Forms control supports scaling. In addition, you cannot scale Windows Forms controls down to a size of 0 pixels.

  • Windows Forms controls support autoscaling, in which the form will automatically resize itself and its controls based on the font size. In a WPF user interface, changing the font size does not resize the entire layout, although individual elements may dynamically resize.

Z-order

In a WPF user interface, you can change the z-order of elements to control overlapping behavior. A hosted Windows Forms control is drawn in a separate HWND, so it is always drawn on top of WPF elements.

A hosted Windows Forms control is also drawn on top of any Adorner elements.

The following sections describe specific aspects of layout behavior when hosting Windows Forms controls in WPF.

Scaling, Unit Conversion, and Device Independence

Whenever the WindowsFormsHost element performs operations involving WPF and Windows Forms dimensions, two coordinate systems are involved: device-independent pixels for WPF and hardware pixels for Windows Forms. Therefore, you must apply proper unit and scaling conversions to achieve a consistent layout.

Conversion between the coordinate systems depends on the current device resolution and any layout or rendering transforms applied to the WindowsFormsHost element or to its ancestors.

If the output device is 96 dpi and no scaling has been applied to the WindowsFormsHost element, one device-independent pixel is equal to one hardware pixel.

All other cases require coordinate system scaling. The hosted control is not resized. Instead, the WindowsFormsHost element attempts to scale the hosted control and all of its child controls. Because Windows Forms does not fully support scaling, the WindowsFormsHost element scales to the degree supported by particular controls.

Override the ScaleChild method to provide custom scaling behavior for the hosted Windows Forms control.

In addition to scaling, the WindowsFormsHost element handles rounding and overflow cases as described in the following table.

Conversion issue

Description

Rounding

WPF device-independent pixel dimensions are specified as double, and Windows Forms hardware pixel dimensions are specified as int. In cases where double-based dimensions are converted to int-based dimensions, the WindowsFormsHost element uses standard rounding, so that fractional values less than 0.5 are rounded down to 0.

Overflow

When the WindowsFormsHost element converts from double values to int values, overflow is possible. Values that are larger than MaxValue are set to MaxValue.

Layout-related Properties

Properties that control layout behavior in Windows Forms controls and WPF elements are mapped appropriately by the WindowsFormsHost element. For more information, see Windows Forms and WPF Property Mapping.

Layout Changes in the Hosted Control

Layout changes in the hosted Windows Forms control are propagated to WPF to trigger layout updates. The InvalidateMeasure method on WindowsFormsHost ensures that layout changes in the hosted control cause the WPF layout engine to run.

Continuously Sized Windows Forms Controls

Windows Forms controls that support continuous scaling fully interact with the WPF layout system. The WindowsFormsHost element uses the MeasureOverride and ArrangeOverride methods as usual to size and arrange the hosted Windows Forms control.

Sizing Algorithm

The WindowsFormsHost element uses the following procedure to size the hosted control:

  1. The WindowsFormsHost element overrides the MeasureOverride and ArrangeOverride methods.

  2. To determine the size of the hosted control, the MeasureOverride method calls the hosted control's GetPreferredSize method with a constraint translated from the constraint passed to the MeasureOverride method.

  3. The ArrangeOverride method attempts to set the hosted control to the given size constraint.

  4. If the hosted control's Size property matches the specified constraint, the hosted control is sized to the constraint.

If the Size property does not match the specified constraint, the hosted control does not support continuous sizing. For example, the MonthCalendar control allows only discrete sizes. The permitted sizes for this control consist of integers (representing the number of months) for both height and width. In cases such as this, the WindowsFormsHost element behaves as follows:

  • If the Size property returns a larger size than the specified constraint, the WindowsFormsHost element clips the hosted control. Height and width are handled separately, so the hosted control may be clipped in either direction.

  • If the Size property returns a smaller size than the specified constraint, WindowsFormsHost accepts this size value and returns the value to the WPF layout system.

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