Description models

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Every add-in and add-in root has to provide a description of the extensibility elements it provides. There are two ways of doing this:

• Using custom attributes: Mono.Addins provides several attributes which can be used to mark classes as extensions or extension points. Attributes can also be used to declare add-in information and dependencies.
• Using an add-in manifest: a manifest is an XML description of an add-in. Attributes are very easy to use, but not all extensibility features that Mono.Addins provides can be expressed using attributes.

It is allowed to use a mix of both approaches to describe an add-in. That is, an add-in could declare simple extensions using attributes, and more complex extensions in a manifest embedded as a resource.

Custom Attributes

Here is a summary of all custom attributes that can be used to describe an extension model. All these attributes belong to the Mono.Addins namespace.

Add-in definition:

Attribute	Target	Description
AddinAttribute	Assembly	Declares that an assembly is an add-in
AddinRootAttribute	Assembly	Declares that an assembly is an add-in host
AddinAuthorAttribute	Assembly	Author of an add-in
AddinDependencyAttribute	Assembly	Declares an add-in dependency
AddinLocalizerGettextAttribute	Assembly	Declares gettext localizer information for the add-in
AddinModuleAttribute	Assembly	Declares an optional module
ImportAddinAssemblyAttribute	Assembly	Declares that an assembly belongs to an add-in. Useful when an add-in is composed by several assemblies
ImportAddinFileAttribute	Assembly	Declares that a file belongs to an add-in. Useful when an add-in is composed by several files

Extension points:

Attribute	Target	Description
ExtensionPointAttribute	Assembly	Declares an extension point
TypeExtensionPointAttribute	Class	Declares a type extension point

Extensions:

Attribute	Target	Description
ExtensionAttribute	Class	Declares a new type extension
ExtensionAttributeAttribute	Class	Declares an attribute value for a type extension (used together with ExtensionAttribute)

Custom extension nodes:

Attribute	Target	Description
ExtensionNodeAttribute	Class	Can be applied to an ExtensionNode subclass to specify some node type metadata such as name or description
ExtensionNodeChildAttribute	Class	Can be applied to an ExtensionNode subclass to specify child node types
NodeAttributeAttribute	Several	Can be applied to a field, property, parameter or class to bind a member to an extension node attribute

XML Manifest

Any declaration that can be done using custom attributes can also be done in a manifest, but not everything that can be declared in a manifest can also be expressed using custom attributes.

An XML add-in manifest is an XML file which contains extension model definitions. Manifests can be deployed as standalone files, or they can be embedded as a resource in assemblies. Manifest files are identified by their extension, which must be ".addin" or ".addin.xml". When embedded in an assembly, a manifest can be split in several .addin files.

Here is a summary of the elements that can be used to describe an extension model:

Element	Description
/Addin	Root element for add-in and add-in root descriptions
/Addin/Runtime/Import	Declares that an assembly or data file belongs to the add-in
/Addin/Runtime/ScanExclude	Declares that a file or directory must be ignored by the add-in scanner
/Addin/Dependencies/Addin	Declares an add-in dependency
/Addin/ExtensionPoint	Declares an extension point
/Addin/ExtensionPoint/ExtensionNode	Describes an extension node of an extension point
/Addin/ExtensionPoint/ConditionType	Declares a condition type local to an extension point
/Addin/Extension	Declares an extension
/Addin/ExtensionNodeSet	Declares an extension node set
/Addin/ConditionType	Declares a global condition type
/Addin/Module	Declares an optional extension module
/Addin/Localizer	Declares a localizer for the add-in