TOPIC
about_methods
SHORT DESCRIPTION
Describes how to use methods to perform actions on objects in Windows
PowerShell.
LONG DESCRIPTION
Windows PowerShell uses structured collections of information, called
objects, to represent the items in data stores or the state of the
computer. For example, when you access a file in Windows PowerShell, you
are not working with the actual file. Instead, you are working with a
FileInfo object, a type of object that acts as the file’s proxy.
Most objects include methods. A method is a set of instructions that
specify a particular action you can take with that object. For instance,
the FileInfo object includes a method called CopyTo, which allows you to
copy the file represented by the object.
To view a list of methods and method definitions associated with a
particular object, you can use the Get-Member cmdlet. However, to use
the cmdlet, the object must already exist in some form, either as
represented through a Variable, as an object created when you specify a
command as an argument to the Get-Member command, or as an object
passed down a pipeline. For example, suppose that the $a Variable has
been assigned a string value, which means that the Variable is
associated with a string object. To view a list of the object’s
methods, enter the following command at the Windows PowerShell command
prompt:
Get-Member -inputobject $a -membertype method
If you want to see which methods and method definitions are associated
with an object that is passed down the pipeline, you would use a
Get-Member command within the pipeline, as shown in the following example:
Get-ChildItem c:\final.txt | Get-Member -membertype method
The most common way to invoke a method is to specify the method name
after an object reference (such as a Variable or expression). You must
separate the object reference and the method with a period. Additionally,
you must use parentheses immediately following the method name to enclose
any arguments that should be passed to the method.
If no arguments are being passed in a method signature, you must still
use a set of empty parentheses.
For example, the following command uses the GetType method to return the
data type associated with the $a string object:
$a.GetType()
The GetType method will return the data type for any object, and a
Variable always represents an object. The type of object depends on the
type of data stored within that Variable.
Every action you take in Windows PowerShell is associated with objects,
whether you are declaring a Variable or combining commands into a pipeline.
As a result, methods can be used in a variety of situations. For example,
you can use a method to take an action on a property value, as shown in
the following command:
(Get-ChildItem c:\final.txt).name.ToUpper()
In this case, the object on which the ToUpper method is being invoked is
the string object associated with the name property. (Note that the
Final.txt file must exist on the root of the C: drive for this example
to work.) The name property is actually a property of the FileInfo object
returned by the Get-ChildItem command. This demonstrates not only the
object-oriented nature of Windows PowerShell, but shows how methods can be
called on any accessible object.
You can achieve the same results as the last example by using a
Variable to store the Get-ChildItem command output, as shown in the
following example:
$a = (Get-ChildItem c:\final.txt).name
$a.ToUpper()
The command again uses the ToUpper method of the string object
associated with the Variable, which contains the file name returned by
the Get-ChildItem command.
In some cases, a method requires an argument to direct the action of
that method. For example, the FileInfo object includes the MoveTo
method, which provides a way to move a file from one location to
another. The method requires an argument that specifies the target
location for the file. The following command demonstrates how to
include that argument:
(Get-ChildItem c:\final.txt).MoveTo(“c:\techdocs\final.txt”)
The Get-ChildItem command returns a FileInfo object for the Final.txt
file and then uses the MoveTo method of that object to initiate the
action and to specify the file’s new location.
To determine the arguments associated with a method, review the
corresponding method definition. A method definition contains one or
more method signatures (also known as overloads in the Microsoft
.NET Framework). A method signature contains the name of a method and zero
or more parameters that you must supply when you call the method. Each
method signature is separated from the prior signature with a comma in the
Get-Member cmdlet display. For example, the CopyTo method of the
FileInfo class contains the following two method signatures:
1. CopyTo(String destFileName)
2. CopyTo(String destFileName, Boolean overwrite)
The first method signature takes the destination file name (including
the path) in which to copy the source file. In the following example,
the first CopyTo method is used to copy Final.txt to the C:\Bin
directory:
(Get-ChildItem c:\final.txt).CopyTo(“c:\bin\final.txt”)
If the file already exists in the destination location, the CopyTo
method fails, and Windows PowerShell reports the following error:
Exception calling “CopyTo” with “1” argument(s): “The file
‘c:\bin\final.txt’ already exists.”.
In the second method signature, you pass the destination file name just
as you did in the first case, but you also pass a Boolean value to
specify whether you want an existing file of the same name in the
destination location to be overwritten, as the following example shows:
(Get-ChildItem c:\final.txt).CopyTo(“c:\bin\final.txt”, $true)
When you pass the Boolean value, you must use the $True Variable, which
is created automatically by Windows PowerShell. The $True Variable
contains the “true” Boolean value. (As you would expect, the $False
Variable contains the “false” Boolean value.)
SEE ALSO
about_objects
Get-Member