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about_Try_Catch_Finally

Updated: June 17, 2009

Applies To: Windows PowerShell 2.0

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>][',' <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_Break
    about_Continue
    about_Scopes
    about_Throw
    about_Trap



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