Define custom types for use with .NET XAML Services

When you define custom types that are business objects or are types that do not have a dependency on specific frameworks, there are certain best practices for XAML you can follow. If you follow these practices, .NET XAML Services and its XAML readers and XAML writers can discover the XAML characteristics of your type and give it appropriate representation in a XAML node stream using the XAML type system. This topic describes best practices for type definitions, member definitions, and CLR attributing of types or members.

Constructor Patterns and Type Definitions for XAML

To be instantiated as an object element in XAML, a custom class must meet the following requirements:

  • The custom class must be public and must expose a parameterless public constructor. (See following section for notes regarding structures.)

  • The custom class must not be a nested class. The extra "dot" in the full-name path makes the class-namespace division ambiguous, and interferes with other XAML features such as attached properties. If an object can be instantiated as an object element, the created object can fill the property element form of any properties that take the object as their underlying type.

You can still provide object values for types that do not meet these criteria, if you enable a value converter. For more information, see Type Converters and Markup Extensions for XAML.


Structures are always able to be constructed in XAML, by CLR definition. This is because a CLR compiler implicitly creates a parameterless constructor for a structure. This constructor initializes all property values to their defaults.

In some cases, the default construction behavior for a structure is not desirable. This might be because the structure is intended to fill values and function conceptually as a union. As a union, the contained values might have mutually exclusive interpretations, and therefore, none of its properties are settable. An example of such a structure in the WPF vocabulary is GridLength. Such structures should implement a type converter so that the values can be expressed in attribute form, by using string conventions that create the different interpretations or modes of the structure values. The structure should also expose similar behavior for code construction through a non-parameterless constructor.


Interfaces can be used as underlying types of members. The XAML type system checks the assignable list and expects that the object that is provided as the value can be assigned to the interface. There is no concept of how the interface must be presented as a XAML type as long as a relevant assignable type supports the XAML construction requirements.

Factory Methods

Factory methods are a XAML 2009 feature. They modify the XAML principle that objects must have parameterless constructors. Factory methods are not documented in this article. See x:FactoryMethod Directive.


Enumerations have XAML native type conversion behavior. Enumeration constant names specified in XAML are resolved against the underlying enumeration type, and return the enumeration value to a XAML object writer.

XAML supports a flags-style usage for enumerations with FlagsAttribute applied. For more information, see XAML Syntax In Detail. (XAML Syntax In Detail is written for the WPF audience, but most of the information in that topic is relevant for XAML that is not specific to a particular implementing framework.)

Member Definitions

Types can define members for XAML usage. It's possible for types to define members that are XAML-usable even if that specific type is not XAML-usable. This is possible because of CLR inheritance. So long as some type that inherits the member supports XAML usage as a type, and the member supports XAML usage for its underlying type or has a native XAML syntax available, that member is XAML-usable.


If you define properties as a public CLR property using the typical CLR get and set accessor patterns and language-appropriate keywording, the XAML type system can report the property as a member with appropriate information provided for XamlMember properties, such as IsReadPublic and IsWritePublic.

Specific properties can enable a text syntax by applying TypeConverterAttribute. For more information, see Type Converters and Markup Extensions for XAML.

In the absence of a text syntax or native XAML conversion and in the absence of further indirection, such as a markup extension usage, the type of a property (TargetType in the XAML type system) must be able to return an instance to a XAML object writer by treating the target type as a CLR type.

If using XAML 2009, x:Reference Markup Extension can be used to provide values if the previous considerations are not met; however, that is more of a usage issue than a type definition issue.


If you define events as a public CLR event, the XAML type system can report the event as a member with IsEvent as true. Wiring the event handlers is not within the scope of .NET XAML Services capabilities; wiring is left to specific frameworks and implementations.


Inline code for methods is not a default XAML capability. In most cases, you do not directly reference method members from XAML, and the role of methods in XAML is only to provide support for specific XAML patterns. x:FactoryMethod Directive is an exception.


CLR design guidelines discourage nonstatic fields. For static fields, you can access static field values only through x:Static Markup Extension; in this case you are not doing anything special in the CLR definition to expose a field for x:Static usages.

Attachable Members

Attachable members are exposed to XAML through an accessor method pattern on a defining type. The defining type itself does not need to be XAML-usable as an object. In fact, a common pattern is to declare a service class whose role is to own the attachable member and implement the related behaviors, but serve no other function such as a UI representation. For the following sections, the placeholder PropertyName represents the name of your attachable member. That name must be valid in the XamlName Grammar.

Be cautious of name collisions between these patterns and other methods of a type. If a member exists that matches one of the patterns, it can be interpreted as an attachable member usage pathway by a XAML processor even if that was not your intention.

The GetPropertyName Accessor

The signature for the GetPropertyName accessor must be:

public static object GetPropertyName(object target)

  • The target object can be specified as a more specific type in your implementation. You can use this to scope the usage of your attachable member; usages outside your intended scope will throw invalid cast exceptions that are then surfaced by a XAML parse error. The parameter name target is not a requirement, but is named target by convention in most implementations.

  • The return value can be specified as a more specific type in your implementation.

To support a TypeConverter enabled text syntax for attribute usage of the attachable member, apply TypeConverterAttribute to the GetPropertyName accessor. Applying to the get instead of the set may seem non-intuitive; however, this convention can support the concept of read-only attachable members that are serializable, which is useful in designer scenarios.

The SetPropertyName Accessor

The signature for the SetPropertyName accessor must be:

public static void SetPropertyName(object target, object value)

  • The target object can be specified as a more specific type in your implementation, with same logic and consequences as described in the previous section.

  • The value object can be specified as a more specific type in your implementation.

Remember that the value for this method is the input coming from the XAML usage, typically in attribute form. From attribute form there must be value converter support for a text syntax, and you attribute on the GetPropertyNames accessor.

Attachable Member Stores

The accessor methods are typically not enough to provide a means to place attachable member values into an object graph, or to retrieve values out of the object graph and serialize them properly. To provide this functionality, the target objects in the previous accessor signatures must be capable of storing values. The storage mechanism should be consistent with the attachable member principle that the member is attachable to targets where the attachable member is not in the members list. .NET XAML Services provides an implementation technique for attachable member stores through the APIs IAttachedPropertyStore and AttachablePropertyServices. IAttachedPropertyStore is used by the XAML writers to discover the store implementation, and should be implemented on the type that is the target of the accessors. The static AttachablePropertyServices APIs are used within the body of the accessors, and refer to the attachable member by its AttachableMemberIdentifier.

Correctly attributing your types, members, and assemblies is important in order to report XAML type system information to .NET XAML Services. Reporting XAML type system information is relevant if either of the following situations apply:

  • You intend your types for use with XAML systems that are directly based on .NET XAML Services XAML readers and XAML writers.
  • You define or use a XAML-utilizing framework that's based on those XAML readers and XAML writers.

For a listing of each XAML-related attribute that's relevant for XAML support of your custom types, see XAML-Related CLR Attributes for Custom Types and Libraries.


Usage of custom types requires that the markup author must map a prefix for the assembly and CLR namespace that contain the custom type. This procedure is not documented in this topic.

Access Level

XAML provides a means to load and instantiate types that have an internal access level. This capability is provided so that user code can define its own types, and then instantiate those classes from markup that is also part of the same user code scope.

An example from WPF is whenever user code defines a UserControl that is intended as a way to refactor a UI behavior, but not as part of any possible extension mechanism that might be implied by declaring the supporting class with public access level. Such a UserControl can be declared with internal access if the backing code is compiled into the same assembly from which it is referenced as a XAML type.

For an application that loads XAML under full trust and uses XamlObjectWriter, loading classes with internal access level is always enabled.

For an application that loads XAML under partial trust, you can control the access level characteristics by using the XamlAccessLevel API. Also, deferral mechanisms (such as the WPF template system) must be able to propagate any access level permissions and preserve them for the eventual run time evaluations; this is handled internally by passing the XamlAccessLevel information.

WPF Implementation

WPF XAML uses a partial-trust access model where if BAML is loaded under partial trust, access is restricted to AssemblyAccessTo for the assembly that is the BAML source. For deferral, WPF uses IXamlObjectWriterFactory.GetParentSettings as a mechanism for passing the access level information.

In WPF XAML terminology, an internal type is a type that is defined by the same assembly that also includes the referencing XAML. Such a type can be mapped through a XAML namespace that deliberately omits the assembly= portion of a mapping, for example, xmlns:local="clr-namespace:WPFApplication1". If BAML references an internal type and that type has internal access level, this generates a GeneratedInternalTypeHelper class for the assembly. If you want to avoid GeneratedInternalTypeHelper, you either must use public access level, or must factor the relevant class into a separate assembly and make that assembly dependent.

See also