Ibrahim just posted something to the PowerShell blog about how to create tied variables in PowerShell. If you extend this approach with script blocks, you have a very powerful dynamic scripting technique.
PS C:\temp> cd \temp
PS C:\temp> New-ScriptVariable.ps1 GLOBAL:lee { $myTestVariable } { $GLOBAL:myTestVariable = 2 * $args[0] }
PS C:\temp> $lee
PS C:\temp> $lee = 10
PS C:\temp> $lee
20
PS C:\temp> New-ScriptVariable.ps1 GLOBAL:today { (Get-Date).DayOfWeek }
PS C:\temp> $today
Wednesday
PS C:\temp> New-ScriptVariable.ps1 GLOBAL:random -Get { Get-Random } -Set { Get-Random -SetSeed $args[0] }
PS C:\temp> $random
1740776676
PS C:\temp> $random
1507521897
PS C:\temp> $random = 10
PS C:\temp> $random
1613858733
PS C:\temp> $random = 10
PS C:\temp> $random
1613858733
He alluded to it in the post – here is the full text of the script:
(Edit 05/17: Updated to make the getters more like PowerShell pipelines: return a single object, or collection of PSObject)
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## New-ScriptVariable.ps1
param($name, [ScriptBlock] $getter, [ScriptBlock] $setter)
Add-Type @"
using System;
using System.Collections.ObjectModel;
using System.Management.Automation;
namespace Lee.Holmes
{
public class PSScriptVariable : PSVariable
{
public PSScriptVariable(string name,
ScriptBlock scriptGetter, ScriptBlock scriptSetter)
: base(name, null, ScopedItemOptions.AllScope)
{
getter = scriptGetter;
setter = scriptSetter;
}
private ScriptBlock getter;
private ScriptBlock setter;
public override object Value
{
get
{
if(getter != null)
{
Collection<PSObject> results = getter.Invoke();
if(results.Count == 1)
{
return results[0];
}
else
{
PSObject[] returnResults = new PSObject[results.Count];
results.CopyTo(returnResults, 0);
return returnResults;
}
}
else { return null; }
}
set
{
if(setter != null) { setter.Invoke(value); }
}
}
}
}
"@
if(Test-Path variable:\$name)
{
Remove-Item variable:\$name -Force
}
$executioncontext.SessionState.PSVariable.Set(
(New-Object Lee.Holmes.PSScriptVariable $name,$getter,$setter)) |