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Understanding XAML (eXtensible Application Markup Language) is a key to creating When you start a new WPF application, you get the following XAML as a. A brief introduction to what XAML (eXtended Application Markup Language) is and what you can use it for. Basic XAML. In the previous chapter, we talked about what XAML is and what you use it for, but how do you create a control in XAML? As you will see from the .
For instance, the Button control allows you to specify the text shown on it between the start and end tags:. NET framework. The same goes for attribute names, which corresponds to the properties of the control.
Here's a button where we define a couple of properties by adding attributes to the tag:.
We set the FontWeight property, giving us bold text, and then we set the Content property, which is the same as writing the text between the start and end tag. However, all attributes of a control may also be defined like this, where they appear as child tags of the main control, using the Control-Dot-Property notation:. The result is exactly the same as above, so in this case, it's all about syntax and nothing else. Attribute syntax is the most streamlined property setting syntax and is the most intuitive syntax to use for developers who have used markup languages in the past.
For example, the following markup creates a button that has red text and a blue background in addition to display text specified as Content.
For these cases, a different syntax known as property element syntax can be used. Generally, the content of that tag is an object element of the type that the property takes as its value. After specifying the content, you must close the property element with an end tag. If an attribute syntax is possible, using the attribute syntax is typically more convenient and enables a more compact markup, but that is often just a matter of style, not a technical limitation.
The following example shows the same properties being set as in the previous attribute syntax example, but this time by using property element syntax for all properties of the Button. One such optimization is that if a particular property takes a collection type, then items that you declare in markup as child elements within that property's value become part of the collection.
In this case a collection of child object elements is the value being set to the collection property. Child elements of that object element are used to set the value of that content property.
For example, Border specifies a content property of Child. The following two Border elements are treated identically. The first one takes advantage of the content property syntax and omits the Border.
Child property element. The second one shows Border. Child explicitly. To enable this, one of the following cases must be true: The class must declare a content property, and that content property must be of a type assignable to a string the type could be Object. The type must declare a type converter, in which case the text content is used as initialization text for that type converter.
This case is less common in practice. The type must be a known XAML language primitive. This is a streamlined and intuitive markup that omits two tags for two different reasons.
Omitted StackPanel. Children property element: StackPanel derives from Panel. Panel defines Panel. Children as its XAML content property. In this case, UIElementCollection actually cannot be instantiated because it does not expose a default constructor, and that is why the UIElementCollection object element is shown commented out. In this case, the attribute's name is the name of the event. For example, you might wonder what the ClickHandler referenced here represents and how it is defined.
Object elements, property elements, and attribute names must all be specified by using the sensitive casing when compared by name to the underlying type in the assembly, or to a member of a type.
XAML language keywords and primitives are also case sensitive. Values are not always case sensitive. Case sensitivity for values will depend on the type converter behavior associated with the property that takes the value, or the property value type. For example, properties that take the Boolean type can take either true or True as equivalent values, but only because the native WPF XAML parser type conversion for string to Boolean already permits these as equivalents. WPF XAML processors and serializers will ignore or drop all nonsignificant white space, and will normalize any significant white space.
This is consistent with the default white-space behavior recommendations of the XAML specification. This behavior is generally only of consequence when you specify strings within XAML content properties. In simplest terms, XAML converts space, linefeed and tab characters into spaces, and then preserves one space if found at either end of a contiguous string. The full explanation of XAML white-space handling is not covered in this topic. This usage directs the XAML processing to escape from the general treatment of attribute values as either a literal string or a string-convertible value.
The most common markup extensions used in WPF application programming are Binding , used for data binding expressions, and the resource references StaticResource and DynamicResource. By using markup extensions, you can use attribute syntax to provide values for properties even if that property does not support an attribute syntax in general.
Markup extensions often use intermediate expression types to enable features such as deferring values or referencing other objects that are only present at run time. For example, the following markup sets the value of the Style property using attribute syntax.
The Style property takes an instance of the Style class, which by default could not be instantiated by an attribute syntax string. But in this case, the attribute references a particular markup extension, StaticResource. When that markup extension is processed, it returns a reference to a style that was previously instantiated as a keyed resource in a resource dictionary.
For a reference listing of the markup extensions that are defined by System.
Xaml and are more widely available for. The basic, native handling of how strings are converted into other object types or primitive values is based on the String type itself, in addition to native processing for certain types such as DateTime or Uri. But many WPF types or members of those types extend the basic string attribute processing behavior, in such a way that instances of more complex object types can be specified as strings and attributes.
Thickness indicates measurements within a nested rectangle and is used as the value for properties such as Margin. By placing a type converter on Thickness , all properties that use a Thickness are easier to specify in XAML because they can be specified as attributes.
An example is Cursor. For more information on how type conversion and its use for attribute syntax is supported, see TypeConverters and XAML. For typical WPF scenarios, you use a root element that has a prominent meaning in the WPF application model for example, Window or Page for a page, ResourceDictionary for an external dictionary, or Application for the application definition. These attributes indicate to a XAML processor which XAML namespaces contain the type definitions for backing types that the markup will reference as elements.
The xmlns attribute specifically indicates the default XAML namespace.
The parent-child hierarchy inherent in XML allows XAML to mimic with greater visual clarity the parent-child hierarchy of user-interface objects. XAML can be easily hand-written by programmers, but also lends itself to be toolable and generated by visual design tools. Of course, there are also disadvantages, mostly related to limitations that are intrinsic to markup languages: XAML cannot contain code.
All event handlers must be defined in a code file. XAML cannot contain loops for repetitive processing. However, several Xamarin. Forms visual objects—most notably ListView —can generate multiple children based on the objects in its ItemsSource collection. XAML cannot contain conditional processing However, a data-binding can reference a code-based binding converter that effectively allows some conditional processing.
XAML generally cannot instantiate classes that do not define a parameterless constructor. However, there is sometimes a way around this restriction. XAML generally cannot call methods.
Again, this restriction can sometimes be overcome. The most important are: Requirements This article assumes a working familiarity with Xamarin. Forms Samples Feedback Send feedback about: This product This page.