Question

If I define the following

[parameter(Mandatory = $true)]
[alias("db")]
[string]$database,

then I get an error

Parameter alias cannot be specified because an alias with the name 'db'
was defined multiple times for the command.

Which is true, since db is already an alias for the universal -Debug parameter.
Is it possible to define this alias without renaming the parameter?

Answer 1

Sorry, you can’t. -Debug is a common parameter, so -Debug and -db are switches that are available on pretty much everything including the functions you write yourself. As the error tells you, it’s already defined.

Even if it were possible to go around undefining built-in aliases, that unexpectantly changes the meaning of a call like test-db -db for someone else who frequently uses -db instead of -Debug. They expect it to enable debugging output, not specify a different parameter.

Consider this function:

function test-db{
  param(
    [parameter(mandatory=$true)]
    [string]$database)
  write-host 'database' $database
  write-debug 'debugging output'
}

Now call it with test-db server, test-db -db server, and test-db server -db. The first doesn’t do write-debug, while the other 2 do, no matter where -db is. You also can’t define a separate parameter [string]$db (or rename $database to $db) because Powershell will give you this error:

Parameter ‘db’ cannot be specified because it conflicts with the parameter alias of the same name for parameter ‘Debug’.

More info on this, per MSDN (updated since old link died):

In addition to using the System.Management.Automation.Aliasattribute attribute, the Windows PowerShell runtime performs partial name matching, even if no aliases are specified. For example, if your cmdlet has a FileName parameter and that is the only parameter that starts with F, the user could enter Filename, Filenam, File, Fi, or F and still recognize the entry as the FileName parameter.