About Throw

Article
09/19/2022

SHORT DESCRIPTION

Describes the Throw keyword, which generates a terminating error.

LONG DESCRIPTION

The Throw keyword causes a terminating error. You can use the Throw keyword to stop the processing of a command, function, or script.

For example, you can use the Throw keyword in the script block of an If statement to respond to a condition or in the Catch block of a Try-Catch-Finally statement. You can also use the Throw keyword in a parameter declaration to make a function parameter mandatory.

The Throw keyword can throw any object, such as a user message string or the object that caused the error.

SYNTAX

The syntax of the Throw keyword is as follows:

throw [<expression>]

The expression in the Throw syntax is optional. When the Throw statement does not appear in a Catch block, and it does not include an expression, it generates a ScriptHalted error.

C:\PS> throw

ScriptHalted
At line:1 char:6
+ throw <<<<
+ CategoryInfo          : OperationStopped: (:) [], RuntimeException
+ FullyQualifiedErrorId : ScriptHalted

If the Throw keyword is used in a Catch block without an expression, it throws the current RuntimeException again. For more information, see about_Try_Catch_Finally.

THROWING A STRING

The optional expression in a Throw statement can be a string, as shown in the following example:

C:\PS> throw "This is an error."

This is an error.
At line:1 char:6
+ throw <<<<  "This is an error."
+ CategoryInfo          : OperationStopped: (This is an error.:String) [], R
untimeException
+ FullyQualifiedErrorId : This is an error.

THROWING OTHER OBJECTS

The expression can also be an object that throws the object that represents the PowerShell process, as shown in the following example:

C:\PS> throw (get-process PowerShell)

System.Diagnostics.Process (PowerShell)
At line:1 char:6
+ throw <<<<  (get-process PowerShell)
+ CategoryInfo          : OperationStopped: (System.Diagnostics.Process (Pow
erShell):Process) [],
RuntimeException
+ FullyQualifiedErrorId : System.Diagnostics.Process (PowerShell)

You can use the TargetObject property of the ErrorRecord object in the $error automatic variable to examine the error.

C:\PS> $error[0].targetobject

Handles  NPM(K)    PM(K)      WS(K) VM(M)   CPU(s)     Id ProcessName
-------  ------    -----      ----- -----   ------     -- -----------
319      26    61016      70864   568     3.28   5548 PowerShell

You can also throw an ErrorRecord object or a Microsoft .NET Framework exception. The following example uses the Throw keyword to throw a System.FormatException object.

C:\PS> $formatError = new-object system.formatexception

C:\PS> throw $formatError

One of the identified items was in an invalid format.
At line:1 char:6
+ throw <<<<  $formatError
+ CategoryInfo          : OperationStopped: (:) [], FormatException
+ FullyQualifiedErrorId : One of the identified items was in an invalid
format.

RESULTING ERROR

The Throw keyword can generate an ErrorRecord object. The Exception property of the ErrorRecord object contains a RuntimeException object. The remainder of the ErrorRecord object and the RuntimeException object vary with the object that the Throw keyword throws.

The RunTimeException object is wrapped in an ErrorRecord object, and the ErrorRecord object is automatically saved in the $Error automatic variable.

USING THROW TO CREATE A MANDATORY PARAMETER

You can use the Throw keyword to make a function parameter mandatory.

This is an alternative to using the Mandatory parameter of the Parameter keyword. When you use the Mandatory parameter, the system prompts the user for the required parameter value. When you use the Throw keyword, the command stops and displays the error record.

For example, the Throw keyword in the parameter subexpression makes the Path parameter a required parameter in the function.

In this case, the Throw keyword throws a message string, but it is the presence of the Throw keyword that generates the terminating error if the Path parameter is not specified. The expression that follows Throw is optional.

function Get-XMLFiles
{
  param ($path = $(throw "The Path parameter is required."))
  dir -path $path\*.xml -recurse |
    sort lastwritetime |
      ft lastwritetime, attributes, name  -auto
}