about_Try_Catch_Finally

TOPIC
	about_Try_Catch_Finally

SHORT DESCRIPTION
	Describes how to use the Try, Catch, and Finally blocks to handle 
	terminating errors.


LONG DESCRIPTION
	Use Try, Catch, and Finally blocks to respond to or handle terminating 
	errors in scripts. The Trap statement can also be used to handle 
	terminating errors in scripts. For more information, see about_Trap.


	A terminating error stops a statement from running. If Windows PowerShell 
	does not handle a terminating error in some way, Windows PowerShell also 
	stops running the function or script using the current pipeline. In other 
	languages, such as C#, terminating errors are referred to as exceptions. 
	For more information about errors, see about_Errors.


	Use the Try block to define a section of a script in which you want Windows 
	PowerShell to monitor for errors. When an error occurs within the Try 
	block, the error is first saved to the $Error automatic variable. Windows 
	PowerShell then searches for a Catch block to handle the error. If the Try 
	statement does not have a  matching Catch block, Windows PowerShell 
	continues to search for an appropriate Catch block or Trap statement in the 
	parent scopes. After a Catch block is completed or if no appropriate Catch 
	block or Trap statement is found, the Finally block is run. If the error 
	cannot be handled, the error is written to the error stream.


	A Catch block can include commands for tracking the failure or for 
	recovering the expected flow of the script. A Catch block can specify which 
	error types it catches. A Try statement can include multiple Catch blocks 
	for different kinds of errors.


	A Finally block can be used to free any resources that are no longer needed 
	by your script. 


	Try, Catch, and Finally resemble the Try, Catch, and Finally keywords used 
	in the C# programming language.


  Syntax
	A Try statement contains a Try block, zero or more Catch blocks, and zero 
	or one Finally block. A Try statement must have at least one Catch block 
	or one Finally block.


	The following shows the Try block syntax:

		try {<statement list>}


	The Try keyword is followed by a statement list in braces. If a 
	terminating error occurs while the statements in the statement list are 
	being run, the script passes the error object from the Try block to an 
	appropriate Catch block. 


	The following shows the Catch block syntax: 

		catch [[<error type>][]] {<statement list>}


	Error types appear in brackets. The outermost brackets indicate the 
	element is optional.


	The Catch keyword is followed by an optional list of error type 
	specifications and a statement list. If a terminating error occurs in the 
	Try block, Windows PowerShell searches for an appropriate Catch block. If 
	one is found, the statements in the Catch block are executed.


	The Catch block can specify one or more error types. An error type is a 
	Microsoft .NET Framework exception or an exception that is derived from a 
	.NET Framework exception. A Catch block handles errors of the specified 
	.NET Framework exception class or of any class that derives from the 
	specified class.


	If a Catch block specifies an error type, that Catch block handles that 
	type of error. If a Catch block does not specify an error type, that 
	Catch block handles any error encountered in the Try block. A Try 
	statement can include multiple Catch blocks for the different specified 
	error types.  


	The following shows the Finally block syntax:

		finally {<statement list>}


	The Finally keyword is followed by a statement list that runs every time 
	the script is run, even if the Try statement ran without error or an 
	error was caught in a Catch statement. 


	Note that pressing CTRL+C stops the pipeline. Objects that are sent to 
	the pipeline will not be displayed as output. Therefore, if you include 
	a statement to be displayed, such as "Finally block has run", it will not 
	be displayed after you press CTRL+C, even if the Finally block ran.


  Catching Errors
	The following sample script shows a Try block with a Catch block:

		try { NonsenseString }
		catch { "An error occurred." }


	The Catch keyword must immediately follow the Try block or another Catch 
	block. 


	Windows PowerShell does not recognize "NonsenseString" as a cmdlet or 
	other item. Running this script returns the following result:

		An error occurred.


	When the script encounters "NonsenseString", it causes a terminating 
	error. The Catch block handles the error by running the statement list 
	inside the block.


  Using Multiple Catch Statements
	A Try statement can have any number of Catch blocks. For example, the 
	following script has a Try block that downloads MyFile.doc, and it 
	contains two Catch blocks:

		try
		{
			 $wc = new-object System.Net.WebClient
			 $wc.DownloadFile("http://www.contoso.com/MyDoc.doc")
	}
		catch [System.Net.WebException],[System.IO.IOException]
		{
			"Unable to download MyDoc.doc from http://www.contoso.com."
	}
		catch
		{
			"An error occurred that could not be resolved."
	}


	The first Catch block handles errors of the System.Net.WebException and 
	System.IO.IOException types. The second Catch block does not specify an 
	error type. The second Catch block handles any other terminating errors 
	that occur.


	Windows PowerShell matches error types by inheritance. A Catch block 
	handles errors of the specified .NET Framework exception class or of any 
	class that derives from the specified class. The following example 
	contains a Catch block that catches a "Command Not Found" error:

		catch [System.Management.Automation.CommandNotFoundException] 
			{"Inherited Exception" }


	The specified error type, CommandNotFoundException, inherits from the 
	System.SystemException type. The following example also catches a Command 
	Not Found error:

		catch [System.SystemException] {"Base Exception" }


	This Catch block handles the "Command Not Found" error and other errors 
	that inherit from the SystemException type.


	If you specify an error class and one of its derived classes, place the 
	Catch block for the derived class before the Catch block for the general 
	class.


  Freeing Resources by Using Finally
	To free resources used by a script, add a Finally block after the Try and 
	Catch blocks. The Finally block statements run regardless of whether the 
	Try block encounters a terminating error. Windows PowerShell runs the 
	Finally block before the script terminates or before the current block 
	goes out of scope. 


	A Finally block runs even if you use CTRL+C to stop the script. A Finally
	block also runs if an Exit keyword stops the script from within a Catch 
	block.


SEE ALSO
	about_Errors
	about_Trap