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White-space processing in XAML

  • Artikkeli

The language rules for XAML state that significant white space must be processed by a XAML processor implementation. This article documents these XAML language rules. It also documents additional white-space handling that is defined by the Windows Presentation Foundation (WPF) implementation of the XAML processor and the XAML writer for serialization.

White-space definition

Consistent with XML, white-space characters in XAML are space, linefeed, and tab. These correspond to the Unicode values 0020, 000A, and 0009 respectively.

White-space normalization

By default the following white-space normalization occurs when a XAML processor processes a XAML file:

  1. Linefeed characters between East Asian characters are removed. See the "East Asian Characters" section later in this topic for a definition of this term.

  2. All white-space characters (space, linefeed, tab) are converted into spaces.

  3. All consecutive spaces are deleted and replaced by one space.

  4. A space immediately following the start tag is deleted.

  5. A space immediately before the end tag is deleted.

"Default" corresponds to the state denoted by the default value of the xml:space attribute.

White space in inner text, and string primitives

The previous normalization rules apply to inner text that is found within XAML elements. After normalization, a XAML processor converts any inner text into an appropriate type as follows:

  • If the type of the property is not a collection but is not directly an Object type, the XAML processor attempts to convert to that type by using its type converter. A failed conversion here causes a compile-time error.

  • If the type of the property is a collection and the inner text is contiguous (no intervening element tags), the inner text is parsed as a single String. If the collection type cannot accept String, this also causes a compile-time error.

  • If the type of the property is Object, the inner text is parsed as a single String. If there are intervening element tags, this causes a compile-time error because the Object type implies a single object (String or otherwise).

  • If the type of the property is a collection, and the inner text is not contiguous, the first substring is converted into a String and added as a collection item, the intervening element is added as a collection item, and finally the trailing substring (if any) is added to the collection as a third String item.

Preserving white space

There are several techniques for preserving white space in the source XAML for eventual presentation that are not affected by XAML processor white-space normalization.

xml:space="preserve": Specify this attribute at the level of the element where white-space preservation is desired. This preserves all white space, which includes the spaces that might be added by code-editing applications to "pretty-print" align elements as a visually intuitive nesting. However, whether those spaces render is determined by the content model for the containing element. Avoid specifying xml:space="preserve" at the root level because most object models do not consider white space as significant regardless of how you set the attribute. Setting xml:space globally may have performance consequences on XAML processing (particularly serialization) in some implementations. It is a better practice to only set the attribute specifically at the level of elements that render white space within strings, or are white-space significant collections.

Entities and non-breaking spaces: XAML supports placing any Unicode entity within a text object model. You can use dedicated entities such as nonbreaking space (  in UTF-8 encoding). You can also use rich text controls that support nonbreaking space characters. You should be cautious if you are using entities to simulate layout characteristics such as indention, because the run-time output of the entities will vary based on a greater number of factors than would the capabilities for producing indention results in a typical layout system, such as proper use of panels and margins. For instance, entities are mapped to fonts and can change size in response to user font selection.

East Asian characters

"East Asian characters" is defined as a set of Unicode character ranges U+20000 to U+2FFFD and U+30000 to U+3FFFD. This subset is also sometimes referred to as "CJK ideographs". For more information, see https://www.unicode.org.

White space and text content models

In practice, preserving white space is only of concern for a subset of all possible content models. That subset is composed of content models that can take a singleton String type in some form, a dedicated String collection, or a mixture of String and other types in an IList or ICollection<T> collection.

White space and text content models in WPF

For illustration purposes, the remainder of this section references particular types that are defined by WPF. The white-space handling features that are described in this article are pertinent to both .NET XAML Services and WPF. To see this behavior in action, you might experiment with some WPF XAML markup, view the results in an object graph, and then serialize back to markup again.

Even for content models that can take strings, the default behavior within these content models is that any white space that remains is not treated as significant. For example, ListBox takes an IList, but the white space (such as linefeeds between each ListBoxItem) is not preserved and not rendered. If you attempt to use linefeeds as separators between strings for ListBoxItem items, it does not work at all; the strings that are separated by the linefeeds are treated as one string and one item.

Those collections that do treat white space as significant are typically part of the flow document model. The primary collection that supports white-space preservation behavior is InlineCollection. This collection class is declared with the WhitespaceSignificantCollectionAttribute; when this attribute is found, the XAML processor will treat white space within the collection as significant. The combination of xml:space="preserve" and white space within a WhitespaceSignificantCollectionAttribute denoted collection is that all white space is preserved and rendered. The combination of xml:space="default" and white space within a WhitespaceSignificantCollectionAttribute causes the initial white-space normalization described earlier, which leaves one space in certain positions, and those spaces are preserved and rendered. Which behavior is desirable is up to you, and you should use xml:space selectively to enable the behavior that you want.

Also, certain inline elements that connote a line break in a flow document model should deliberately not introduce an extra space even in a white-space significant collection. For example, the LineBreak element has the same purpose as the <BR/> tag in HTML, and for readability in markup, typically a LineBreak is separated from any subsequent text by an authored linefeed. That linefeed should not be normalized to become a leading space in the subsequent line. To enable that behavior, the class definition for the LineBreak element applies the TrimSurroundingWhitespaceAttribute, which is then interpreted by the XAML processor to mean that white space surrounding LineBreak is always trimmed.

See also