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ABOUT REF
Short description
Describes how to create and use a reference type variable. You can use
reference type variables to permit a function to change the value of a
variable that is passed to it.
Long description
You can pass variables to functions _by reference_ or _by value_.
When you pass a variable _by value_, you are passing a copy of the data.
In the following example, the function changes the value of the variable
passed to it. In PowerShell, integers are value types so they are passed by
value. Therefore, the value of $var is unchanged outside the scope of the
function.
Function Test($data)
{
$data = 3
}
$var = 10
Test -data $var
$var
10
In the following example, a variable containing a Hashtable is passed to a
function. Hashtable is an object type so by default it is passed to the
function _by reference_.
When passing a variable _by reference_, the function can change the data
and that change persists after the function executes.
Function Test($data)
{
$data.Test = "New Text"
}
$var = @{}
Test -data $var
$var
Name Value
---- -----
Test New Text
The function adds a new key-value pair that persists outside of the
function's scope.
Writing functions to accept reference parameters
You can code your functions to take a parameter as a reference, regardless
of the type of data passed. This requires that you specify the parameters
type as System.Management.Automation.PSReference, or [ref].
When using references, you must use the Value property of the
System.Management.Automation.PSReference type to access your data.
Function Test([ref]$data)
{
$data.Value = 3
}
To pass a variable to a parameter that expects a reference, you must type
cast your variable as a reference.
[!NOTE] The brackets and parenthesis are BOTH required.
$var = 10
Test -data ([ref]$var)
$var
3
Passing references to .NET methods
Some .NET methods may require you to pass a variable as a reference. When
the method's definition uses the keywords in, out, or ref on a parameter,
it expects a reference.
[int] | Get-Member -Static -Name TryParse
Name MemberType Definition
---- ---------- ----------
TryParse Method static bool TryParse(string s, [ref] int result)
The TryParse method attempts to parse a string as an integer. If the method
succeeds, it returns $true, and the result is stored in the variable you
passed BY REFERENCE.
PS> $number = 0
PS> [int]::TryParse("15", ([ref]$number))
True
PS> $number
15
References and scopes
References allow the value of a variable in the parent scope to be changed
within a child scope.
# Create a value type variable.
$i = 0
# Create a reference type variable.
$iRef = [ref]0
# Invoke a scriptblock to attempt to change both values.
&{$i++;$iRef.Value++}
# Output the results.
"`$i = $i;`$iRef = $($iRef.Value)"
$i = 0;$iRef = 1
Only the reference type's variable was changed.
See also
about_Variables
about_Environment_Variables
about_Functions
about_Script_Blocks
about_Scopes