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BUILDING AND MANIPULATING OBJECTS
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or do anything else that we can do with a regular object. But there s more to this than simply selecting from the existing set of members. For example, say we want to add a new field minute to these objects. This will be a calculated field as follows:
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PS (9) > dir | %{$_.lastwritetime.minute} 55 51 56 54
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In other words, it will be the minute at which the file was last written. We attach this field by passing a specially constructed hashtable describing the member to SelectObject. This hashtable has to have two members: name and expression (which can be shortened to n and e for brevity). The name is the name to call the property, and the expression is the scriptblock used to calculate the value of the field. This definition will look like:
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@{n="minute";e={$_.lastwritetime.minute}}
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Let s use it in the pipeline:
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PS (11) > dir | select-object name,length, >> @{n="minute";e={$_.lastwritetime.minute}} >> Name ---a.txt b.txt c.txt d.txt Length -----98 42 102 66 minute -----55 51 56 54
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As intended, the result has three fields, including the synthetic minute property we specified with the hashtable. Using Get-Member to see what the object looks like:
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PS (12) > dir | select-object name,length, >> @{n="minute";e={$_.lastwritetime.minute}} | gm >> TypeName: System.Management.Automation.PSCustomObject Name ---Equals GetHashCode GetType ToString Length minute Name MemberType ---------Method Method Method Method NoteProperty NoteProperty NoteProperty Definition ---------System.Boolean Equals(Object obj) System.Int32 GetHashCode() System.Type GetType() System.String ToString() System.Int64 Length=98 System.Management.Automation.PSObjec... System.String Name=a.txt
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we see that there are now three NoteProperty members on the objects that were output. 232
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SCRIPTBLOCKS AND OBJECTS
Having looked at the nice way to add members to objects and build synthetic objects, let s dig into the actual plumbing of the PowerShell type system. In the next section, we ll look at what s happening under the covers.
A CLOSER LOOK AT THE TYPE-SYSTEM PLUMBING
Earlier in this chapter, we said that the core of the PowerShell type system was the PSObject type. This type is used to wrap other objects, providing adaptation and inspection capabilities, as well as a place to attach synthetic members. We ve used Get-Member to explore objects and used the Add-Member and Select-Object cmdlets to extend and create objects. In fact, you can do all of this directly by using the PSObject class itself. And there s one thing you can t do without understanding PSObject: wrapping or shadowing an existing property. In this technique, the synthetic property calls the base property that it s hiding. (Don t worry; this is less esoteric than it sounds. A simple example will clarify what we re talking about here.)
AUTHOR S NOTE
If you ve done much object-oriented programming, this concept is similar to creating an override to a virtual method that calls the overridden method on the base class. The difference here is that it s all instance-based; there is no new type involved.
Let s look at PSObject in more detail. First, let s look at the properties on this object:
PS (1) > [psobject].getproperties() | %{$_.name} Members Properties Methods ImmediateBaseObject BaseObject TypeNames
From the list, we see some obvious candidates of interest. But how does one get at these members, given that the whole point of PSObject is to be invisible The answer is that there s a special property attached to all objects in PowerShell called (surprise) PSObject. Let s look at this. First we need a test object to work on. We ll use get-item to retrieve the DirectoryInfo object for the C: drive.
PS (2) > $f = get-item c:\ PS (3) > $f Directory: Mode ---d--hs LastWriteTime ------------5/29/2006 3:11 PM Length Name ------ ---C:\
Now let s look at the PSObject member attached to this object.
A CLOSER LOOK AT THE TYPE-SYSTEM PLUMBING
PS (4) > $f.psobject : {PSPath, PSParentPath, PSChildName, PSDriv e...} Properties : {PSPath, PSParentPath, PSChildName, PSDriv e...} Methods : {get_Name, get_Parent, CreateSubdirectory, Create...} ImmediateBaseObject : C:\ BaseObject : C:\ TypeNames : {System.IO.DirectoryInfo, System.IO.FileSy stemInfo, System.MarshalByRefObject, Syste m.Object} Members
Right away you see a wealth of information: all of the properties we saw on the PSObject type, populated with all kinds of interesting data. First let s look at the TypeNames member:
PS (6) > $f.psobject.typenames System.IO.DirectoryInfo System.IO.FileSystemInfo System.MarshalByRefObject System.Object
This member contains the names of all of the types in the inheritance hierarchy for a DirectoryInfo object. (These types are all documented in the .NET class library documentation that is part of the Microsoft Developers Network [MSDN] collection. See http://msdn.microsoft.com for more information.) We'll look at the Properties member next. This is a collection that contains all of the properties defined by this type. Let s get information about all of the properties that contain the pattern name :
PS (7) > $f.psobject.properties | {$_.name -match "name"} MemberType IsSettable IsGettable Value TypeNameOfValue Name IsInstance MemberType Value IsSettable IsGettable TypeNameOfValue Name IsInstance : : : : : : : : : : : : : : NoteProperty True True C:\ System.String PSChildName True Property C:\ False True System.String Name True
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