Powershell忽略Add-Type，除非将其直接复制并粘贴到控制台

Question

我有以下代码。目的是在按下按钮后更改Label的文本

Add-Type -AssemblyName System.Windows.Forms
Add-Type -AssemblyName System.Drawing

class MyForm : System.Windows.Forms.Form {
    MyForm($mystuff) {
        #Do-Stuff
        $this.Add_Load( $this.MyForm_Load )
    }

    $MyForm_Load = {
        $mlabel = [System.Windows.Forms.Label]::new()
        $mlabel.Name = "status"
        $mlabel.Text = "enabled"

        $mbutton = [System.Windows.Forms.Button]::new() 
        $mbutton.Text = "toggle state"

        $mbutton.Location = [System.Drawing.Point]::new(100,100)
        $mbutton.Add_Click( $this.mbutton_click )

        $this.Controls.Add($mlabel)
        $this.Controls.Add($mbutton)
    }

    $mbutton_click = {
        if ($this.Parent.Controls["status"].Text -eq "enabled"){
            $this.Parent.Controls["status"].Text = "disabled"
        }
        else{
            $this.Parent.Controls["status"].Text = "enabled"
        }
    }
}

$foo = [MyForm]::new("test")
$foo.ShowDialog()

由于以下错误，我无法直接从脚本运行代码：

At C:\Users\wakatana\oop_vs_proc\forms2_oop.ps1:4 char:16
+ class MyForm : System.Windows.Forms.Form {
+                ~~~~~~~~~~~~~~~~~~~~~~~~~
Unable to find type [System.Windows.Forms.Form].
At C:\Users\wakatana\oop_vs_proc\forms2_oop.ps1:11 char:20
+         $mlabel = [System.Windows.Forms.Label]::new()
+                    ~~~~~~~~~~~~~~~~~~~~~~~~~~
Unable to find type [System.Windows.Forms.Label].
At C:\Users\wakatana\oop_vs_proc\forms2_oop.ps1:15 char:21
+         $mbutton = [System.Windows.Forms.Button]::new()
+                     ~~~~~~~~~~~~~~~~~~~~~~~~~~~
Unable to find type [System.Windows.Forms.Button].
At C:\Users\wakatana\oop_vs_proc\forms2_oop.ps1:18 char:30
+         $mbutton.Location = [System.Drawing.Point]::new(100,100)
+                              ~~~~~~~~~~~~~~~~~~~~
Unable to find type [System.Drawing.Point].
    + CategoryInfo          : ParserError: (:) [], ParseException
    + FullyQualifiedErrorId : TypeNotFound

在VS代码中也显示相同的错误，并用红色下划线标出。但是，当我启动powershell控制台并复制粘贴脚本的第一行（Add-Type -AssemblyName System.Windows.Forms），然后从此控制台运行脚本时，脚本运行良好。有什么问题吗？