about_Functions_Advanced_Methods
Short description
Describes how functions that specify the CmdletBinding
attribute can use the
methods and properties that are available to compiled cmdlets.
Long description
Functions that specify the CmdletBinding
attribute can access additional
methods and properties through the $PSCmdlet
variable. These methods include
the following methods:
- The same input-processing methods available to all functions.
- The
ShouldProcess
andShouldContinue
methods that are used to get user feedback before an action is performed. - The
ThrowTerminatingError
method for generating error records. - Several
Write
methods that return different types of output.
All the methods and properties of the PSCmdlet class are available to advanced functions. For more information, see System.Management.Automation.PSCmdlet.
For more information about the CmdletBinding
attribute, see
about_Functions_CmdletBindingAttribute. For the
CmdletBindingAttribute class, see
System.Management.Automation.Cmdlet.CmdletBindingAttribute.
Input processing methods
The methods described in this section are referred to as the input processing
methods. For functions, these three methods are represented by the begin
,
process
, and end
blocks of the function.
You aren't required to use any of these blocks in your functions. If you don't
use a named block, then PowerShell puts the code in the end
block of the
function. However, if you use any of these named blocks, or define a
dynamicparam
block, you must put all code in a named block.
The following example shows the outline of a function that contains a begin
block for one-time preprocessing, a process
block for multiple record
processing, and an end
block for one-time post-processing.
Function Test-ScriptCmdlet
{
[CmdletBinding(SupportsShouldProcess=$True)]
Param ($Parameter1)
begin{}
process{}
end{}
}
Note
These blocks apply to all functions, not just functions that use the
CmdletBinding
attribute.
begin
This block is used to provide optional one-time preprocessing for the function. The PowerShell runtime uses the code in this block once for each instance of the function in the pipeline.
process
This block is used to provide record-by-record processing for the function. You
can use a process
block without defining the other blocks. The number of
process
block executions depends on how you use the function and what input
the function receives.
The automatic variable $_
or $PSItem
contains the current object in the
pipeline for use in the process
block. The $input
automatic variable
contains an enumerator that's only available to functions and script blocks.
For more information, see about_Automatic_Variables.
- Calling the function at the beginning, or outside of a pipeline, executes the
process
block once. - Within a pipeline, the
process
block executes once for each input object that reaches the function. - If the pipeline input that reaches the function is empty, the
process
block does not execute.- The
begin
andend
blocks still execute.
- The
Important
If a function parameter is set to accept pipeline input, and a process
block isn't defined, record-by-record processing will fail. In this case,
your function will only execute once, regardless of the input.
When you create a function that accepts pipeline input and uses
CmdletBinding
, the process
block should use the parameter variable you
defined for pipeline input instead of $_
or $PSItem
. For example:
function Get-SumOfNumbers {
[CmdletBinding()]
param (
[Parameter(Mandatory, Position=0, ValueFromPipeline)]
[int[]]$Numbers
)
begin { $retValue = 0 }
process {
foreach ($n in $Numbers) {
$retValue += $n
}
}
end { $retValue }
}
PS> Get-SumOfNumbers 1,2,3,4
10
PS> 1,2,3,4 | Get-SumOfNumbers
10
end
This block is used to provide optional one-time post-processing for the function.
Confirmation methods
ShouldProcess
This method is called to request confirmation from the user before the function
performs an action that would change the system. The function can continue
based on the Boolean value returned by the method. This method can only be
called only from within the Process{}
block of the function. The
CmdletBinding
attribute must also declare that the function supports
ShouldProcess
(as shown in the previous example).
For more information about this method, see System.Management.Automation.Cmdlet.ShouldProcess.
For more information about how to request confirmation, see Requesting Confirmation.
ShouldContinue
This method is called to request a second confirmation message. It should be
called when the ShouldProcess
method returns $true
. For more information
about this method, see
System.Management.Automation.Cmdlet.ShouldContinue.
Error methods
Functions can call two different methods when errors occur. When a
non-terminating error occurs, the function should call the WriteError
method,
which is described in the Write
methods section. When a terminating error
occurs and the function can't continue, it should call the
ThrowTerminatingError
method. You can also use the Throw
statement for
terminating errors and the Write-Error cmdlet for non-terminating errors.
For more information, see System.Management.Automation.Cmdlet.ThrowTerminatingError.
Write methods
A function can call the following methods to return different types of output.
Notice that not all the output goes to the next command in the pipeline. You
can also use the various Write
cmdlets, such as Write-Error
.
WriteCommandDetail
For information about the WriteCommandDetails
method, see
System.Management.Automation.Cmdlet.WriteCommandDetail.
WriteDebug
To provide information that can be used to troubleshoot a function, make the
function call the WriteDebug
method. The WriteDebug
method displays debug
messages to the user. For more information, see
System.Management.Automation.Cmdlet.WriteDebug.
WriteError
Functions should call this method when non-terminating errors occur and the function is designed to continue processing records. For more information, see System.Management.Automation.Cmdlet.WriteError.
Note
If a terminating error occurs, the function should call the ThrowTerminatingError method.
WriteObject
The WriteObject
method allows the function to send an object to the next
command in the pipeline. In most cases, WriteObject
is the method to use when
the function returns data. For more information, see
System.Management.Automation.PSCmdlet.WriteObject.
WriteProgress
For functions with actions that take a long time to complete, this method
allows the function to call the WriteProgress
method so that progress
information is displayed. For example, you can display the percent completed.
For more information, see
System.Management.Automation.PSCmdlet.WriteProgress.
WriteVerbose
To provide detailed information about what the function is doing, make the
function call the WriteVerbose
method to display verbose messages to the
user. By default, verbose messages aren't displayed. For more information, see
System.Management.Automation.PSCmdlet.WriteVerbose.
WriteWarning
To provide information about conditions that may cause unexpected results, make the function call the WriteWarning method to display warning messages to the user. By default, warning messages are displayed. For more information, see System.Management.Automation.PSCmdlet.WriteWarning.
Note
You can also display warning messages by configuring the $WarningPreference
variable or by using the Verbose
and Debug
command-line options. for more
information about the $WarningPreference
variable, see
about_Preference_Variables.
Other methods and properties
For information about the other methods and properties that can be accessed
through the $PSCmdlet
variable, see
System.Management.Automation.PSCmdlet.
For example, the ParameterSetName property allows you to see the parameter set that's being used. Parameter sets allow you to create a function that performs different tasks based on the parameters that are specified when the function is run.