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SCRIPTBLOCKS AND OBJECTS
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InvokeScript Method NewScriptBlock Method ToString Method
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System.Collections.ObjectModel.Coll... System.Management.Automation.Script... System.String ToString()
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The interesting methods in this list are ExpandString(), InvokeScript(), and NewScriptBlock(). These methods are covered in the next few sections. The ExpandString() method The ExpandString() method lets you perform the same kind of variable interpolation that the PowerShell runtime does in scripts. Here s an example. First we set $a to a known quantity:
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PS (2) > $a = 13
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Next we create a variable $str that will display the value of $a.
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PS (3) > $str='a is $a'
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Since the variable was assigned using single-quotes, no string expansion took place. We verify this by displaying the string:
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PS (4) > $str a is $a
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Now call the ExpandString() method, passing in $str:
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PS (5) > $ExecutionContext.InvokeCommand.ExpandString($str) a is 13
and it returns the string with the variable expanded into its value. The InvokeScript() method The next method to look at is InvokeScript(). This method does the same thing that the Invoke-Expression cmdlet does. It takes its argument and evaluates it like a script. Call this method passing in the string 2+2
PS (7) > $ExecutionContext.InvokeCommand.InvokeScript("2+2") 4
and it will return 4. The NewScriptBlock() method The final method to look at is the NewScriptBlock() method. Like InvokeScript(), this method takes a string, but instead of executing it, it returns a scriptblock object that represents the compiled script. Let s use this method to turn the string '1..4 | foreach {$_ * 2}' into a scriptblock.
PS (8) > $sb = $ExecutionContext.InvokeCommand.NewScriptBlock( >> '1..4 | foreach {$_ * 2}') >>
BUILDING CODE AT RUNTIME
We saved this scriptblock into a variable, so let s look at it. Since the ToString() on a scriptblock is the code of the scriptblock, we just see the code that makes up the body of the scriptblock.
PS (9) > $sb 1..4 | foreach {$_ * 2}
Now let s execute the scriptblock using the "&" call operator.
PS (10) > & $sb 2 4 6 8
The scriptblock executed, printing out the even numbers from 4 to 8.
AUTHOR S NOTE
Many people have asked why we (the PowerShell team) don t allow you to simply cast a string to a scriptblock. The reason is that we want to make the system resilient against code injection attacks. We want to minimize the number of places where executable code can be injected into the system, and we particularly want code creation to be an explicit act. Casts are more easily hidden, leading to accidental code injections, especially when the system may prompt for a string. We don t want those user-provided strings to be converted into code without some kind of check. See chapter 13 for more extensive discussions about security.
Creating functions using the function: drive The final way to create a scriptblock is actually a side-effect of creating elements in the function drive. Earlier we saw that you can create a named function by assigning a scriptbock to a name in the function drive:
PS (1) > $function:foo = {"Hello there"} PS (2) > foo Hello there
You could also use the New-Item cmdlet to do this:
PS (3) > new-item function:foo -value {"Hi!"} New-Item : The item at path 'foo' already exists. At line:1 char:9 + new-item <<<< function:foo -value {"Hi!"}
We received an error because the function already exists, so let s use the -force parameter to overwrite the existing definition:
PS (4) > new-item function:foo -value {"Hi!"} -force CommandType ----------Function Name ---foo Definition ---------"Hi!"
SCRIPTBLOCKS AND OBJECTS
New-Item returns the item created, so we can see that the function has been changed. But that s using scriptblocks. What happens if we pass in strings The interpreter will compile these strings into scriptblocks and then assign the scriptblock to the name. Here s an example where the body of the function is determined by the expanded string.
PS (5) PS (6) PS (7) PS (8) 30 PS (9) 5*6 > > > > $x=5 $y=6 $function:foo = "$x*$y" foo
> $function:foo
The variables $x and $y expanded into the numbers 5 and 6 in the string, so the resulting scriptblock was
{5*6}
Now let s define another function using foo, but adding some more text to the function.
PS (10) > new-item function:bar -value "$function:foo*3" CommandType ----------Function PS (11) > bar 90 Name ---bar Definition ---------5*6*3
In the expanded string, $function:foo expanded into 5*6 so the new function bar was assigned a scriptblock
{5*6*3}
This concludes our survey of ways to build new code at runtime in PowerShell. There are even more ways to build code using .NET s Reflection.Emit classes. We ll cover those techniques in chapter 11. For now, though, we ve covered everything about metaprogramming in PowerShell.
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