Using Custom Validation Attributes

Beginning in PowerShell 5, you can create your own attributes, i.e. custom validators. They can be applied to variables (and parameters), and once a value is assigned that does not match the validator, an exception is raised.

Here is an example for a path validator. When you apply it to a variable, only valid file paths can be applied to the variable:

class ValidatePathExistsAttribute : System.Management.Automation.ValidateArgumentsAttribute
{
    # the value to be checked surfaces in $path and must be of type [object]
    [void]Validate([object]$path, [System.Management.Automation.EngineIntrinsics]$engineIntrinsics)
    {
        # if anything is wrong with the value, throw an exception
        if([string]::IsNullOrWhiteSpace($path))
        {
            Throw [System.ArgumentNullException]::new()
        }
        if(-not (Test-Path -Path $path))
        {
            Throw [System.IO.FileNotFoundException]::new()
        }        

        # if NO exception was thrown, the value is accepted
    }
}
#endregion


[ValidatePathExists()][string]$Path = "c:\windows"
$Path = "c:\test123"

Whenever you assign a path that does not exist, PowerShell will *not* assign it and instead keep the existing value.

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