PowerShell function returns duplicates in DataTable

Question

I have a PowerShell 5.0 function which I'm trying to use to return a data table that is used in some downstream activities. However, it seems to return duplicates everytime I place this into a variable as so:

function GetTable {

    $tbl = New-Object "System.Data.DataTable" 
    $tbl.Columns.Add("testNum").DataType = [string]

    for($i=0; $i -lt 3; $i++){
            $tbl.Rows.Add($i)
    }
    Write-Host $tbl.Rows.Count
    return $tbl
}

$test = GetTable
$test.Rows.Count

This outputs:

3
6

Why does the function duplicate the data when returning the data table?

Martin Brandl · Accepted Answer · 2018-02-10 14:08:29Z

4

That is because the Add method returns the added element so it also gets to the pipeline. You can simply fix that by pipe the result to Out-Null:

function GetTable {

    $tbl = New-Object "System.Data.DataTable" 
    $tbl.Columns.Add("testNum").DataType = [string]

    for($i=0; $i -lt 3; $i++){
            $tbl.Rows.Add($i) | Out-Null
    }
    Write-Host $tbl.Rows.Count
    return $tbl
}

$test = GetTable
$test.Rows.Count

answered Feb 10, 2018 at 14:08

Martin Brandl

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2 Comments

codedawg82 Over a year ago

Would this be the same case for the Columns.Add method? Or is the fact that the datatype is being specified to it prevent that from happening?

Martin Brandl Over a year ago

In PowerShell, everything that you call that returns something and you don't assign to a variable or pipe to anything gets put to the pipeline and will be returned even you don't use the return keyword. So if the Columns.Add method returns anything the answer is yes.

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