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• PowerShell: Return values from a function through reference parameters

  Thursday, March 26, 2009

  .NET PowerShell

  3 Comments

  PowerShell has a special [ref] keyword to specify which parameters are used in a fuction to return values. It's usage is not directly clear however.

  If the type of the variable to update is a value type, use the [ref] keyword in the declaration of the function parameter, this specifies that a "pointer" to the variable will be passed.

  To pass the "pointer" to the variable, use ([ref]$variable) to create the pointer, and pass this as parameter. To set the value of the value variable pointed to by the "pointer", use the .Value field.

  Simple example to show what [ref] does:

  C:\Program Files\PowerGUI> $zz = "hoi"
  C:\Program Files\PowerGUI> $xz = [ref]$zz
  C:\Program Files\PowerGUI> $xz
  Value
  -----
  hoi

  C:\Program Files\PowerGUI> $xz.Value = "dag"
  C:\Program Files\PowerGUI> $zz
  dag 

  This is only required for value types, not is the type is a referece type like for example Hashtable.

  Some sample code:

  function fn
  {
  param
  (
  [ref]$arg1,
  [ref]$arg2,
  $arg3
  )
  
  $arg1.Value = 1
  $arg2.Value = "overwrite"
  $arg3.key = "overwrite hash value"
  }
  
  $x = 0
  $y = "original"
  $z = @{"key" = "original hashvalue"}
  $x
  $y
  $z.key
  fn -arg1 ([ref]$x) -arg2 ([ref]$y) -arg3 $z
  $x
  $y
  $z.key