Extended Type System Overview
PowerShell uses its PSObject object to extend the types of objects in two ways. First, the PSObject object provides a way to show different views of specific object types. This is referred to as showing an adapted view of an object. Second, the PSObject object provides a way to add members to existing object. Together, by wrapping an existing object, referred to as the base object, the PSObject object provides an extended type system (ETS) that script and cmdlet developers can use to manipulate .NET objects within the shell.
Cmdlet and Script Development Issues
ETS resolves two fundamental issues:
First, some .NET Objects do not have the necessary default behavior for acting as the data between cmdlets.
- Some .NET objects are "meta" objects (for example: WMI Objects, ADO objects, and XML objects) whose members describe the data they contain. However, in a scripting environment it is the contained data that is most interesting, not the description of the contained data. ETS resolves this issue by introducing the notion of Adapters that adapt the underlying .NET object to have the expected default semantics.
- Some .NET Object members are inconsistently named, provide an insufficient set of public members, or provide insufficient capability. ETS resolves this issue by introducing the ability to extend the .NET object with additional members.
Second, the PowerShell scripting language is typeless in that a variable does not need to be declared of a particular type. That is, the variables a script developer creates are by nature typeless. However, the PowerShell system is "type-driven" in that it depends on having a type name to operate against for basic operations such as outputting results or sorting.
Therefore a script developer must have the ability to state the type of one of their variables and build up their own set of dynamically typed "objects" that contain properties and methods and can participate in the type-driven system. ETS solves this problem by providing a common object for the scripting language that has the ability to state its type dynamically and to add members dynamically.
Fundamentally, ETS resolves the issue mentioned previously by providing the PSObject object, which acts as the basis of all object access from the scripting language and provides a standard abstraction for the cmdlet developer.
For the cmdlet developers, ETS provides the following support:
- The abstractions to work against objects in a generic way using the PSObject object. ETS also provides the ability to drill past these abstractions if required.
- The mechanisms to create a default behavior for formatting, sorting, serialization, and other system manipulations of their object type using a well-known set of extended members.
- The means to operate against any object using the same semantics as the script language using a LanguagePrimitives object.
- The means to dynamically "type" a hash table so that the rest of the system can operate against it effectively.
For the script developers, ETS provides the following support:
- The ability to reference any underlying object type using the same syntax (
- The ability to access beyond the abstraction provided by the PSObject object (such as accessing only adapted members, or accessing the base object itself).
- The ability to define well-known members that control the formatting, sorting, serialization, and other manipulations of an object instance or type.
- The means to name an object as a specific type and thus control the inheritance of its type-based members.
- The ability to add, remove, and modify extended members.
- The ability to manipulate the PSObject object itself if required.
The PSObject class
The PSObject object is the basis of all object access from the scripting language and provides a standard abstraction for the cmdlet developer. It contains a base-object (a .NET object) and any instance members (members, specifically extended members, that are present on a particular object instance while not necessarily on other objects of the same type). Depending on the type of the base-object, the PSObject object might also provide implicit and explicit access to adapted members as well as any type-based extended members.
The PSObject object provides the following mechanisms:
- The ability to construct a PSObject with or without a base-object.
- The ability to access of all members of each constructed PSObject object through a common lookup algorithm and the ability to override that algorithm when required.
- The ability to get and set the type-names of the constructed PSObject objects so that scripts and cmdlets can reference similar PSObject objects by the same type-name, regardless of the type of their base-object.
How to Construct a PSObject
The following list describes ways to create a PSObject object:
- Calling the PSObject .#ctor constructor creates a new PSObject object with a base-object of PSCustomObject. A base-object of this type indicates that the PSObject object has no meaningful base-object. However, a PSObject object with this type of base-object does provide a property bag that cmdlet developers can find helpful by adding extended-members.
Developers can also specify the object type-name, which allows this object to share its extended-members with other PSObject objects of the same type-name.
Calling the PSObject .#ctor(System.Object) constructor creates a new PSObject object with a base-object of type System.Object.
In this case, the type-name for the created object is a collection of the derivation hierarchy of the base-object. For example, the type-name for the PSObject that contains a ProcessInfo base-object would include the following names:
Calling the PSObject .AsPSObject(System.Object) method creates a new PSObject object based on a supplied object.
If the supplied object is of type System.Object, the supplied object is used as the base-object for the new PSObject object. If the supplied object is already a PSObject object, the supplied object is returned as is.
Base, adapted, and extended members
Conceptually, ETS uses the following terms to show the relationship between the original members of the base-object and those members added by PowerShell. For more information about the specific types of members that are used by the PSObject object, see PSObject class.
If the base-object is specified when constructing the PSObject objects, then the members of the base-object are made available through the Members property.
When a base-object is a meta-object, one that contains data in a generic fashion whose properties "describe" their contained data, ETS adapts those objects to a view that allows for direct access to the data through adapted members of the PSObject object. Adapted members and base-object members are accessed through the Members property.
In addition to the members made available from the base-object or those adapted members created by PowerShell, a PSObject may also define extended members that extend the original base-object with additional information that is useful in the scripting environment.
For example, all the core cmdlets provided by PowerShell, such as the Get-Content and Set-Content cmdlets, take a path parameter. To ensure that these cmdlets, and others, can work against objects of different types, a Path member can be added to those objects so that they all state their information in a common way. This extended Path member ensures that the cmdlets can work against all those types even though there base class might not have a Path member.
Extended members, adapted members, and base-object members are all accessed through the Members property.