Methods

 

SHORT DESCRIPTION

Using methods to perform actions on objects in the Windows PowerShell

 

LONG DESCRIPTION

PowerShell uses structured collections of information, called objects, to represent items in data stores or the state of the computer. For example, when you access a file in PowerShell, you're not working with the actual file, but instead you're 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 PowerShell command prompt:

 

 

If you want to see what 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:

 

 

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. In addition, 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:

 

 

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 PowerShell is associated with objects, whether you're 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:

 

 

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 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:

 

 

The command again uses the ToUpper() method of the string object associated with the variable, which contains the filename 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:

 

 

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 .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 filename (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:

 

 

If the file already exists in the destination location, the CopyTo method will fail and PowerShell will report the following error:

 

 

In the second method signature, you pass the destination filename 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:

 

 

When you pass the boolean value, you must use the $true variable, which is created automatically by PowerShell. The $true variable contains the "true" boolean value. (As you would expect, the $false variable contains the "false" boolean value.)

 

SEE ALSO

For information about objects, enter the following command:

 

 

For information about the Get-Member Cmdlet, enter the following command: