The Extending Type System

Adapted views of objects are defined by Windows PowerShell cmdlets. These cmdlets expose a view of an existing object where only a selected set of its members are directly accessible (an adapted view of an object does not change the existing object).

The following illustration shows the relationship between a PSObject object, its base-object, the adapter that defines the adapted view, and any extended members of the base object. Notice that in this example, method M2 and property P2 are blocked by the adapter. These members are not directly accessible through the Members, Methods, and Properties properties of the PSObject object, however they are indirectly accessible through the BaseObject property.

The following are some of the .NET types whose selected members are accessed through adapters.

• ManagementBaseObject

• DirectoryEntry

• DataRow

• DataRowView

• XmlNode

The following is an example of the adapted view of the ManagementObject object that represents the current computer. The first line obtains the object by invoking the Get-WmiObject cmdlet. The second line invokes the Get-Member cmdlet that displays

PS>$p = get-wmiobject -class win32_computersystem

PS>$p | get-member

TypeName: System.Management.ManagementObject#root\cimv2\Win32_ComputerSystem

Name MemberType Definition

---- ---------- ----------

JoinDomainOrWorkgroup Method System.Management.ManagementBaseObject JoinDomainOrWorkgroup(System.Stri

Rename Method System.Management.ManagementBaseObject Rename(System.String Name, System

SetPowerState Method System.Management.ManagementBaseObject SetPowerState(System.UInt16 Power

UnjoinDomainOrWorkgroup Method System.Management.ManagementBaseObject UnjoinDomainOrWorkgroup(System.St

AdminPasswordStatus Property System.UInt16 AdminPasswordStatus {get;set;}

AutomaticResetBootOption Property System.Boolean AutomaticResetBootOption {get;set;}

AutomaticResetCapability Property System.Boolean AutomaticResetCapability {get;set;}

BootOptionOnLimit Property System.UInt16 BootOptionOnLimit {get;set;}

Delete ScriptMethod System.Object Delete();

GetType ScriptMethod System.Object GetType();

Put ScriptMethod System.Object Put();

Here are two things to notice about this output

• Not all the members of the base Win32_computerSystem object are displayed by this adapted view. For example, the cmdlet is designed to not display the AutomaticManagedProfile property of the base object.

• The last three lines display ScriptMethods that have been added by the cmdlet.

To display these properties, Windows PowerShell uses an instance of PSObject created by the cmdlet that keeps a reference to the original object. The original WMI object is accessible through the BaseObject property.

The following example shows how to display the members of the original object.

PS>$p = get-wmiobject -class win32_computersystem

PS>$p | get-member

TypeName: System.Management.Automation.PSMemberSet

Name MemberType Definition

---- ---------- ----------

Members can be added to existing objects so that they can be used consistently with other objects within the shell. The following illustration shows the relationship between a PSObject object, its base class, and the extended members added to the base object. Note that, when adding members, if you add an extended member with the same name as a member of the base object, the extended member will take precedence over the base object member.

Windows PowerShell allows the addition of properties and methods to existing objects using a number of ETS classes, as listed in the following table. Members can be added to the objects programmatically, by defining the object using a custom MyProductName.Types.ps1xml file, and through the command line. The members are represented in the types file using the constants defined in the PSMemberTypes enumeration, for example, "CodeMethod". For more information, see Extending Object Types and Formatting.

Note:
Members of a base object are indicated by the PSProperty, PSMethod, and PSParameterizedProperty classes. These types of members cannot be added to an existing object.

The following .NET code example shows how to add a note property programmatically to an object.

PSObject psObj = new PSObject("some base object");
psObj.Properties.Add(new PSNoteProperty("MyProperty","Value"));

The following example shows to add a note property at the command line.

$a = "some base object"
$a = $a | add-member Note* MyProperty Value

Windows PowerShell uses the PSObject class to allow access to serialized objects written to streams. The Windows PowerShell serialization import mechanism uses PSObject to store the object properties. Then the Windows PowerShell runtime can interact with the stored properties using the associated API.

In the next example, serialized output is provided by the Get-Process cmdlet, which generates a .NET Process object. When the object is deserialized (imported from a serialized state), Windows PowerShell can still access most of its properties.

Note:
Windows PowerShell can access the properties, but not the methods, of a deserialized object.

PS> $a=get-process ps
PS> $a

Handles  PM(K)   WS(K) VS(M)     Id ProcessName
-------  -----   ----- -----     -- -----------
    270  18536   25188   127    804 ps
    514  55288   56944   175   2356 ps

PS> get-process ps |export-clixml abc.xml
PS> import-clixml abc.xml

Handles  PM(K)   WS(K) VS(M)     Id ProcessName
-------  -----   ----- -----     -- -----------
    270  18536   25188   127    804 ps
    562  55300   56956   175   2356 ps

Windows PowerShell supports the creation of new objects based on the PSObject class. These objects will reflect in a customized types file, as indicated in Extending Object Types and Formatting.