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How parameter validation works
Validation attributes declare the values a parameter accepts. During parameter binding, PowerShell checks supplied input against those rules before invoking the function or cmdlet. Invalid input produces an error instead of being passed to the function body. See Microsoft Learn’s Validating Parameter Input overview and about_Functions_Advanced_Parameters.
How do I validate a parameter in PowerShell? Add the appropriate attribute inside the parameter declaration, before the parameter’s type. For example, [ValidateRange(1, 90)] [int] $Days makes the allowed numeric interval part of the parameter contract. Validation attributes can also constrain variables; for a typed variable, place the attribute before the type so validation follows type conversion rather than occurring first.
Choose an attribute for the shape of the rule
| Input rule | Attribute | What it checks |
|---|---|---|
| One of a closed list of choices | ValidateSet |
Accepts only listed values and supports tab completion. |
| Numeric minimum and maximum | ValidateRange |
Requires a value within the specified bounds. |
| Minimum and maximum text length | ValidateLength |
Checks the string’s character count against the bounds. |
| Required text format | ValidatePattern |
Matches a regular expression. Use ^ and $ anchors when the entire string, rather than a substring, must match. |
| Minimum and maximum collection size | ValidateCount |
Checks the number of arguments or elements. |
| A custom acceptance condition | ValidateScript |
Runs a script block against the candidate value. |
| Reject null | ValidateNotNull |
Rejects $null, subject to type conversion behavior. |
| Reject null and empty input | ValidateNotNullOrEmpty |
Rejects $null, an empty string, and an empty array. |
| Restrict the drive in a path | ValidateDrive |
Checks the path’s drive portion; it does not establish that the remainder of the path exists. |
Microsoft’s advanced-parameter reference and attribute type reference document these constraints. Select the narrowest attribute that expresses the actual contract; a format check, for example, is not a substitute for checking whether a path exists.
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Common parameter declarations
Limit a parameter to named choices
Use ValidateSet when the accepted values form a genuinely closed list. It rejects other values and provides tab completion in supported command-line use. When the choices need to be generated dynamically, the documentation describes IValidateSetValuesGenerator, available starting with PowerShell 6.0; check the target environment before using that interface. See Microsoft Learn’s advanced-parameter guidance and argument completion reference.
Set numeric bounds or text length
ValidateRange communicates numeric limits directly in the declaration. ValidateLength is for a string whose character count must remain within specified bounds. Use these when the bounds are stable parts of the parameter’s contract rather than incidental checks buried in the function body.
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Check a text pattern
ValidatePattern applies a regular expression. Regex patterns can match only part of a string unless you anchor them. For whole-value validation, use a pattern shaped like ^[A-Z]{3}d{4}$; adapt the expression to the actual format rather than treating this illustrative pattern as a universal identifier rule.
Constrain a collection’s size
ValidateCount is appropriate when the number of supplied elements itself is part of the rule. It is distinct from validation of the contents: use an additional suitable constraint if individual values also need to meet a requirement.
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Use ValidateScript only for custom rules
ValidateScript evaluates a script block with the candidate value available as $_. A result of $false fails validation, as does an exception. For example, a script condition can check a property or a relationship between values that a simpler built-in attribute cannot express. Keep custom validation focused on acceptance; the function body can still handle the work of using an accepted value.
There are two important constraints: $null cannot be passed to ValidateScript for validation, and when the attribute is applied to a collection, every element must satisfy the script block. These behaviors are documented in Microsoft Learn’s about_Functions_Advanced_Parameters.
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Handle null, empty, and whitespace deliberately
These inputs are not interchangeable. ValidateNotNull rejects null, but a null value assigned to a string parameter can be converted to an empty string. If neither null nor empty strings are meaningful, ValidateNotNullOrEmpty states that intent more clearly; it also rejects an empty array.
An empty string and a whitespace-only string are different: the latter contains characters. The current advanced-parameter documentation also describes ValidateNotNullOrWhiteSpace for rejecting whitespace-only text. Because available attributes can vary by PowerShell version, verify support in the version where the function will run before relying on it.
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Put attributes before types and treat defaults separately
For typed parameters and variables, put the validation attribute before the type, as in [ValidateNotNullOrEmpty()] [string] $OutputPath. Microsoft documents that placing a validation attribute after the type can make validation happen before type conversion and lead to unexpected failures.
Parameter validation applies to supplied input; a default value in the declaration is not validated as though it had been supplied by the caller. If a default must satisfy a rule, do not infer that simply from attaching an attribute to the parameter—ensure the value is established and checked through an appropriate design.
Putting the attributes together
This illustrative function combines a closed set, a numeric range, and a nonempty string requirement:
function Get-Report {
param(
[Parameter(Mandatory)]
[ValidateSet('Daily', 'Weekly', 'Monthly')]
[string] $Period,
[ValidateRange(1, 90)]
[int] $Days = 7,
[ValidateNotNullOrEmpty()]
[string] $OutputPath
)
# Function body
}Period is limited to declared report intervals, Days has numeric bounds, and an empty output path is rejected when supplied. The default of 7 is not proof that parameter binding validated the default; defaults are not validated as supplied parameter input.
Quick Recap
Practical checks before relying on a rule
- Use a finite set only when new values should actually be rejected; use dynamic set generation when the allowed choices need to be computed.
- For patterns, decide whether a substring match is sufficient or the complete input must match, and anchor the regex for the latter.
- Decide separately whether null, empty string, empty collection, or whitespace-only text is invalid.
- For custom script validation, account for null behavior and whether the candidate can be a collection whose elements are checked individually.
- Confirm version support for newer attributes or dynamic value generation on the PowerShell installations that will run the function.
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