In PowerShell Form.Show() does not work right, but Form.ShowDialog() does

There are ways to make this work, but nothing is worth spending five hours explaining on an open forum. There are other free, shrink-wrapped ways to do this on powershell. Most notably with the free WPF powershell toolkit: Show-UI at http://showui.codeplex.com/ (previously known as WPK and/or PowerBoots - they are merged now.)

First Answer Why it appends.

In a Windows graphic program the thread which create a Window must loop in a message pump in order to redistribute (translate) messages coming from the user action to events in his Windows.

In a modal window, the modal code that handles the window display runs its own message pump loop and doesn't return until the window is closed. That's why the code after ShowDialog() won't execute until the window is closed.

Show(), just ask to show the Window, but if there is no pump loop to manage the messages coming from user action, it just freezes.

Second a simple way to have two threads

The CmdLet start-job use another thread from the pool allocated by Powershell so it makes the dialog non-modal, and it does not freeze.

function goForm
{
  [void][reflection.assembly]::LoadWithPartialName("System.Windows.Forms")

  $file = (get-item 'C:\temp\jpb.png')
  #$file = (get-item "c:\image.jpg")

  $img = [System.Drawing.Image]::Fromfile($file);

  # This tip from http://stackoverflow.com/questions/3358372/windows-forms-look-different-in-powershell-and-powershell-ise-why/3359274#3359274
  [System.Windows.Forms.Application]::EnableVisualStyles();
  $form = new-object Windows.Forms.Form
  $form.Text = "Image Viewer"
  $form.Width = $img.Size.Width;
  $form.Height =  $img.Size.Height;
  $pictureBox = new-object Windows.Forms.PictureBox
  $pictureBox.Width =  $img.Size.Width;
  $pictureBox.Height =  $img.Size.Height;

  $pictureBox.Image = $img;
  $form.controls.add($pictureBox)
  $form.Add_Shown( { $form.Activate() } )
  $form.ShowDialog()
}

Clear-Host

start-job $function:goForm

$name = Read-Host "What is you name"
Write-Host "your name is $name"