Strange behavior in PowerShell function returning DataSet/DataTable

Question

This is driving me crazy. I have a library I source from multiple scripts, which contains the following function:

function lib_open_dataset([string] $sql) {
         


        

Answer 1

You can use the comma operator to wrap the rows collection in an array so that when the array is unrolled you wind up with the original rows collection e.g.:

function lib_open_table([string] $sql) {
    $ds = lib_open_dataset $sql    
    return ,$ds.Tables[0]
}

Essentially you can't prevent PowerShell from unrolling arrays/collections. The best you can do is workaround that behavior by wrapping the array/collection within another, single element array.

Answer 2

PowerShell special-cases the DataTable internally. It does not implement any of the usual suspect interfaces like ICollection, IList or IEnumerable which normally trigger the unrolling. You can dig into this a bit with:

PS> $dt = new-object data.datatable
PS> $dt -is [collections.ienumerable]
False

Yet:

PS> $e = [management.automation.languageprimitives]::GetEnumerator($dt)
PS> $e.gettype()

IsPublic IsSerial Name                                     BaseType
-------- -------- ----                                     --------
False    False    RBTreeEnumerator                         System.ValueType

-Oisin

Answer 3

Oh yes, I've been struggeling with this one too until I got this post..(tnxs Keith !)

2 things you need to indeed focus on a) prepend your returned object with the comma indeed b) when you're filling your adaptor, make sure to either assign the outcome to a (disposalble) variable or do an Out-Null

I didn't do the Out-Null and even with a prepended comma, I kept getting a collection back (item 0= number of rows from the query, item1= the datatable) Drove my a bit crazy until I picked the Out-null parameter out.

Very weird IMHO, as I'm asking specifically to return the datatable but kept getting the collection back, even with the "," in front

function  Oracleconnection
{
  process
  {
  trap
    {
      Write-Host "error occured on oracle connection"
      Write-Host $_
      continue
    }
    [System.Reflection.Assembly]::LoadWithPartialName(“System.Data.OracleClient”) | out-null
    $connection = new-object system.data.oracleclient.oracleconnection( `
    "Data Source=(DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=myhost.host)(PORT=1800)) `
    (CONNECT_DATA=(SERVICE_NAME=myservicename)));User Id=myid;Password=mypassword;");

    $query = "SELECT country, asset FROM table "
    $set = new-object system.data.dataset
    $adapter = new-object system.data.oracleclient.oracledataadapter ($query, $connection)
    $adapter.Fill($set) | Out-Null
    $table = new-object system.data.datatable
    $table = $set.Tables[0]
    return ,$table
  }
}