Gets or sets the width of a FrameworkElement.
Assembly: System.Windows (in System.Windows.dll)
Dependency property identifier field: WidthProperty
Several of the FrameworkElement derived classes are also derived from Shape. Not all of the Shape classes typically use Height or to specify their appearance, and instead use specific properties that might define a set of points. In this case a Height or is calculated, but is not typically set directly. See Shapes and Drawing.
Custom classes might have similar considerations where the class might have properties that are more meaningful for specifying dimensions than are Height or . In this case Height or will both still be available as members and are settable, even though the desirable behavior might be to not enable Height or setting. In critical cases, it might be worth shadowing the Height and properties to prevent them from being set directly by consumers of the class.
TextBlock is an example of a container object, where the container displays text within it. The size of that text can influence the size of the rendered TextBlock; therefore, TextBlock might not be honoring the specified Height or . For container objects such as TextBlock, you should check two additional properties that expose that calculated size, ActualHeight and ActualWidth.
Negative values for are not permitted. Non-integral values for are technically permitted. Values are preserved as non-integral values if set that way in XAML or code, but should generally be avoided and are normally rounded by the default layout rounding behavior. See Layout Rounding.
In addition to the validation check, there is a nondeterministic upper value bound for (this is a very large number, larger than Single.MaxValue but smaller than Double.MaxValue). Do not set to a value that is significantly larger than the maximum size of any possible visual display, or you may exceed this nondeterministic upper bound.
"Auto" Layout and Double.NaN
The default value of Height and is not 0; it is Double.NaN. Height and support the ability to be an unset "Auto" value. Because Height and are double values, Double.NaN is used as a special value to represent this "Auto" behavior. The layout system interprets the "Auto" value to generally mean that the object should be sized to the available size in layout, instead of to a specific pixel value.
If you want the "Auto" behavior for an object when it is used in layout, leave Height and unset at their Double.NaN default value. If you have previously set values and want to reenable the "Auto" behavior, set Height and specifically to Double.NaN.
Although not a common practice except for perhaps in visual state templates, you can also explicitly set Height and attributes in XAML to be the string Auto instead of a double value. Setting Auto in Silverlight XAML is a special behavior of the Silverlight 5 XAML parser, not a TypeConverter behavior.
For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.