{"id":8716,"date":"2015-02-11T06:00:00","date_gmt":"2015-02-11T14:00:00","guid":{"rendered":"http:\/\/www.sapien.com\/blog\/?p=8716"},"modified":"2015-02-11T06:21:44","modified_gmt":"2015-02-11T14:21:44","slug":"parameter-validation-affects-parameter-variables","status":"publish","type":"post","link":"https:\/\/dev.sapien.com\/blog\/2015\/02\/11\/parameter-validation-affects-parameter-variables\/","title":{"rendered":"Parameter validation affects parameter variables"},"content":{"rendered":"

Follow @juneb_get_help<\/a><\/p>\n

In a Google+ discussion this week, Edi Prinz<\/a> demonstrated an effect of the ValidationRange<\/strong> parameter attribute that surprised many of us. When you validate a parameter value, the validation rules are enforced on the parameter variable — and that enforcement persists on the variable for the entire function scope.<\/p>\n

\"EdiPrinz\"<\/a><\/p>\n

For example, this function has a Value parameter. It uses the ValidateRange<\/b> parameter attribute to allow only parameter values between 3 and 8, inclusive.<\/p>\n

function Test-Validation {\r\n    Param (\r\n        [ValidateRange(3,8)]\r\n        [Int32]\r\n        $Value\r\n    )\r\n\r\n    $Value = 3 * $Value\r\n    $Value\r\n}<\/pre>\n
Let’s call the function with a value of 4 (between 3 and 8) for the parameter.<\/p>\n
Test-Validation \u2013Value 4\r\n\r\nThe variable cannot be validated because the value \r\n12 is not a valid value for the Value variable.\r\nAt C:\\ps-test\\Test-ParameterValidation.ps1:7 char:5\r\n+     $Value = 3 * $Value\r\n+     ~~~~~~~~~~~~~~~~~~~\r\n    + CategoryInfo          : MetadataError: (:) [], ValidationMetadataException\r\n    + FullyQualifiedErrorId : ValidateSetFailure<\/span>\r\n\r\n4\r\n<\/pre>\n
 <\/p>\n
What’s happening here? Why is there an error when the Value parameter has a value of 4?<\/p>\n
When you call the Test-Validation function with a value of 2 for the Value parameter, it returns a parameter validation error (“Cannot validate argument on parameter…”), as expected, because a parameter value of 2 is not between 3 and 8 inclusive.<\/p>\n
PS C:\\ps-test> Test-Validation -Value 2\r\n\r\nTest-Validation : Cannot validate argument on parameter 'Value'. \r\nThe 2 argument is less than the minimum allowed range of 3. \r\nSupply an argument that is greater than or equal to 3 and then try the command again.\r\nAt line:1 char:24\r\n+ Test-Validation -Value 2\r\n+\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 ~\r\n+ CategoryInfo\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 : InvalidData: (:) [Test-Validation], \r\nParameterBindingValidationException\r\n+ FullyQualifiedErrorId : ParameterArgumentValidationError,Test-Validation<\/span><\/pre>\n
 <\/p>\n
When you enter a parameter value that’s within the allowed range (such as 4), you don’t get a parameter<\/em> validation error. Instead, the command that saves the product of “3 * $Value” (12) in the $Value variable causes a variable<\/em> validation error (“…variable cannot be validated…”) error because 12 is not between 3 and 8 inclusive.<\/p>\n
$Value = 3 * $Value<\/pre>\n
The variable<\/em> value failed the range test, even though the parameter<\/em> value satisfied it. Notice that the final $Value command returns a value of 4, because the command to assign a value of 12 failed.<\/p>\n
PS C:\\ps-test> Test-Validation -Value 4\r\nThe variable cannot be validated because the \r\nvalue 12 is not a valid value for the Value variable.\r\nAt C:\\ps-test\\Test-Validation.ps1:11 char:2\r\n+\u00a0\u00a0\u00a0\u00a0 $Value = 3 * $Value\r\n+\u00a0\u00a0\u00a0\u00a0 ~~~~~~~~~~~\r\n+ CategoryInfo\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 : MetadataError: (:) [], ValidationMetadataException\r\n+ FullyQualifiedErrorId : ValidateSetFailure<\/span>\r\n\r\n4\r\n<\/pre>\n
 <\/p>\n
Surprisingly, the validation that we set on the Value parameter is enforced on the $Value variable, even though we didn’t set any validation on the variable.<\/p>\n
Let’s see why it happens, when it happens, and how to fix it.<\/p>\n
 <\/p>\n
Why: Parameters and parameter variables<\/h2>\n
Why does this happen?<\/p>\n
In Windows PowerShell, function parameters are automatically converted to variables so you can use them in function commands. For example, the -Value<\/strong> parameter automatically becomes a $Value<\/strong> variable. If Windows PowerShell didn’t do this for you, you might need to save the parameter value in local variable explicitly.<\/p>\n
function Get-Triple\r\n{\r\n    Param ([Int32]$Value)\u00a0\u00a0 #<-\u2014 This parameter...\r\n\r\n    $Value = 3 * $Value\u00a0\u00a0\u00a0\u00a0 #<\u2014- becomes this variable.\r\n}<\/pre>\n
 <\/p>\n
But, in the Test-Validation function script block, we forgot about the parameter-ness of this parameter variable and just thought of it as a variable. And that’s fine. Most of the time. But not when you’re using parameter validation attributes.<\/p>\n
 <\/p>\n
Where: What’s the scope of this effect?<\/h2>\n
Windows PowerShell applies parameter validation to the parameter variable in the script.<\/p>\n