try
{
    [System.Xml.XmlWriter] $writer = [System.Xml.XmlWriter]::Create('c:\some.xml')
}
finally
{
    $writer.Dispose()
}

错误：

Method invocation Failed because
[System.Xml.XmlWellFormedWriter]
doesn’t contain a method named
‘Dispose’.

另一方面：

$writer -is [IDisposable]
 # True

我该怎么办？