Powershell 默认下拉值

我有一个脚本，用户可以从下拉列表中选择选项。但如果用户没有选择任何内容，我就会收到错误。即使用户未输入值，如何设置返回的默认值。

这是脚本

########################

# Edit This item to change the DropDown Values

[array]$DropDownArray = "c", "d", "share"

# This Function Returns the Selected Value and Closes the Form

function Return-DropDown {
 $script:Choice = $DropDown.SelectedItem.ToString()
 $Form.Close()
}

function selectShare{
    [void] [System.Reflection.Assembly]::LoadWithPartialName("System.Windows.Forms")
    [void] [System.Reflection.Assembly]::LoadWithPartialName("System.Drawing")


    $Form = New-Object System.Windows.Forms.Form

    $Form.width = 300
    $Form.height = 150
    $Form.Text = ”DropDown”

    $DropDown = new-object System.Windows.Forms.ComboBox
    $DropDown.Location = new-object System.Drawing.Size(100,10)
    $DropDown.Size = new-object System.Drawing.Size(130,30)

    ForEach ($Item in $DropDownArray) {
     [void] $DropDown.Items.Add($Item)
    }

    $Form.Controls.Add($DropDown)

    $DropDownLabel = new-object System.Windows.Forms.Label
    $DropDownLabel.Location = new-object System.Drawing.Size(10,10) 
    $DropDownLabel.size = new-object System.Drawing.Size(100,40) 
    $DropDownLabel.Text = "Select shared folder to deploy SEP"
    $Form.Controls.Add($DropDownLabel)

    $Button = new-object System.Windows.Forms.Button
    $Button.Location = new-object System.Drawing.Size(100,50)
    $Button.Size = new-object System.Drawing.Size(100,20)
    $Button.Text = "Select an Item"
    $Button.Add_Click({Return-DropDown})
    $form.Controls.Add($Button)

    $Form.Add_Shown({$Form.Activate()})
    [void] $Form.ShowDialog()


    return $script:choice
}

$share = selectShare
write-host $share

如果用户不从下拉列表中选择，则会出现错误

You cannot call a method on a null-valued expression.
At D:\Script\comp.ps1:10 char:2
+  $script:Choice = $DropDown.SelectedItem.ToString()
+  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
    + FullyQualifiedErrorId : InvokeMethodOnNull

为什么不直接做：

$DropDown.SelectedItem = $DropDown.Items[0]

在你展示表格之前？或者将其设置为 = 无论您的默认项目是什么。

Edit:或者更好的是，设置一个值检查函数并在单击“确定”时调用该函数以确保所有字段都具有有效值。