为什么此Powershell函数有时返回列表，而有时返回XmlElement？

出于某种原因，如果我的XML文件仅包含一个计算机节点，则以下Read-TestControllerXml powershell函数将返回System.Xml.XmlElement类型的对象，但是如果有多台计算机，它将返回一个列表（这是我一直希望的）节点。

XML文件的示例如下：

<test_controller>
    <defaults>
        <nunit path='C:\Program Files (x86)\NUnit 2.6.2\bin\nunit-console.exe' />
        <nunit_results local_path='C:\Test_Results' remote_path='C:\Test_Results' />
        <powershell path='C:\windows\system32\WindowsPowerShell\v1.0\powershell.exe' />
        <psexec path='C:\PSTools\PsExec.exe' />
    </defaults>

    <platform os='windows' version='2012' cpu='x64'>
      <computer hostname='VM-2012R2-03' alias='standalone-01' configuration='standalone' image='VM-2012R2-03:Clean' />
    </platform>
</test_controller>

这是powershell函数：

####################################################################
# Parses the TestController.xml file and returns a list of Computer XML objects.
#
# Syntax:
#     Read-TestControllerXml [-TestControllerXml] <path to TestController.xml> [<CommonParameters>]
#
# Returns:  An array of [Xml] nodes for each of the <computer> tags in TestController.xml.
####################################################################
function Read-TestControllerXml
{
    [CmdletBinding()]
    Param
    (
        [parameter(Mandatory=$true)]
        [string] $TestControllerXml
    )

    $ErrorActionPreference = "Stop"

    # Read the TestController XML configuration file
    $TestController = [xml] (Get-Content $TestControllerXml)

    Write-Verbose "Selecting computer nodes..."
    $computerNodes = $TestController.SelectNodes("//test_controller/platform/computer")
    Write-Verbose "Selecting default nodes..."
    $defaultNodes = $TestController.SelectSingleNode("//test_controller/defaults")

    $computers = @()

    foreach ($computerNode in $computerNodes)
    {
        Write-Verbose "***** Before adding default attributes *****"
        Write-Verbose "$($computerNode.OuterXml)"

        # Add default attributes to the node
        foreach ($dnode in $defaultNodes.ChildNodes)
        {
            if ($dnode.NodeType -eq "Comment") { continue } # Skip comments.

            # Search for the node items in the defaults section
            $cnode = $computerNode

            if ($dnode.Name -ne $computerNode.Name)
            {
                Write-Verbose "Selecting '$($dnode.Name)' nodes..."
                $cnode = $computerNode.SelectSingleNode($dnode.Name)
            }

            # Append the default nodes that do not exist in the computer specific sections  
            if ($cnode -eq $null)
            {
                Write-Verbose "*** cnode != null  cnode.OuterXml is: '$($cnode.OuterXml)'"
                Write-Verbose "Appending default '$($dnode.Name)' node that don't exist in the computer nodes..."
                $cnode=$computerNode.AppendChild($TestController.ImportNode($dnode, $true))
            }
            # Append the default attributes that do not exist in the computer specific sections  
            else
            {
                Write-Verbose "Appending default attributes that don't exist in computer nodes..."
                foreach ($attribute in $dnode.Attributes)
                {
                    if (-not $cnode.HasAttribute($attribute.Name))
                    {
                        Write-Verbose "Adding attribute '$($attribute.Name)=$($attribute.Value)' to the computer node."
                         $cnode.SetAttribute($attribute.Name, $attribute.Value)
                    }
                }
            }
        }

        Write-Verbose "***** After adding default attributes *****"
        Write-Verbose "$($computerNode.OuterXml)"
        Write-Verbose "************************************************************"

        $computers += $computerNode
    }

    return $computers
}