Powershell and SQL parameters. If empty string, pass DBNull - sql

Question

I got this parameter:
$objDbCmd.Parameters.Add("#telephone", [System.Data.SqlDbType]::VarChar, 18) | Out-Null;
$objDbCmd.Parameters["#telephone"].Value = $objUser.Telephone;
Where the string $objUser.Telephone can be empty. If it's empty, how can I convert it to [DBNull]::Value?
I tried:
if ([string]:IsNullOrEmpty($objUser.Telephone)) { $objUser.Telephone = [DBNull]::Value };
But that gives me the error:
Exception calling "ExecuteNonQuery" with "0" argument(s): "Failed to convert parameter value from a ResultPropertyValueCollection to a String."
And if I convert it to a string, it inserts an empty string "", and not DBNull.
How can this be accomplished?
Thanks.

Answer 1

In PowerShell, you can treat null/empty strings as a boolean.
$x = $null
if ($x) { 'this wont print' }
$x = ""
if ($x) { 'this wont print' }
$x = "blah"
if ($x) { 'this will' }
So.... having said that you can do:
$Parameter.Value = $(if ($x) { $x } else { [DBNull]::Value })
But I'd much rather wrap this up in a function like:
function CatchNull([String]$x) {
if ($x) { $x } else { [DBNull]::Value }
}

Answer 2

I don't know about powershell, but in C# I would do something like this:
if ([string]::IsNullOrEmpty($objUser.Telephone))
{
$objDbCmd.Parameters["#telephone"].Value = [DBNull]::Value;
}
else
{
$objDbCmd.Parameters["#telephone"].Value = $objUser.Telephone;
}

Answer 3

Always append +"" at the end of db values...
$command.Parameters["#EmployeeType"].Value= $ADResult.EmployeeType + ""

Answer 4

Many years later, let me clarify:
Josh's answer shows a helpful simplification for testing strings for emptiness (relying on PowerShell's implicit to-Boolean conversion[1]), but it is unrelated to Tommy's (the OP's) problem.
Instead, the error message
"Failed to convert parameter value from a ResultPropertyValueCollection to a String."
implies that it is the non-null case that caused the problem, because $objDbCmd.Parameters["#telephone"].Value expects either a string value or [DBNull]::Value, whereas $objUser.Telephone is of type [ResultPropertyValueCollection], i.e. a collection of values.
Thus, in the non-null case, a string value must be assigned, which must be derived from the collection; one option is to take the first collection element's value, another would be to join all values with a separator to form a single string, using, e.g., [string]::Join(';', $objUser.Telephone) or, if joining the elements with spaces is acceptable (not a good idea with multiple phone numbers), simply with "$($objUser.Telephone)".[2]
Detecting an empty collection via [string]:IsNullOrEmpty() actually worked, despite the type mismatch, due to how PowerShell implicitly stringifies collections when passing a value to a [string] typed method parameter.[2]
Similarly, using implicit to-Boolean conversion works as expected with collections too: an empty collection evaluates to $false, a non-empty one to $true (as long as there are either at least two elements or the only element by itself would be considered $true[1])
Therefore, one solution is to use the first telephone number entry:
$objDbCmd.Parameters["#telephone"].Value = if ($objUser.Telephone) {
$objUser.Telephone[0].ToString() # use first entry
} else {
[DBNull]::Value
}
Note: If $objUser.Telephone[0] directly returns a [string], you can omit the .ToString() call.
In PowerShell v7+ you can alternatively shorten the statement via a ternary conditional:
$objDbCmd.Parameters["#telephone"].Value =
$objUser.Telephone ? $objUser.Telephone[0].ToString() : [DBNull]::Value
[1] For a comprehensive summary of PowerShell's automatic to-Boolean conversions, see the bottom section of this answer.
[2] When implicitly converting a collection to a string, PowerShell joins the stringified elements of a collection with a single space as the separator by default; you can override the separator with the automatic $OFS variable, but that is rarely done in practice; e.g., array 'foo', 'bar' is converted to 'foo bar'; note that this conversion does not apply when you call the collection's .ToString() method explicitly, but it does apply inside expandable (interpolating) strings, e.g., "$array".