process.start problem in for each loop

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VB

.NET

I am stuck in a problem in calling a command-line application through a button-click code.

the application is ffmpeg.
my application is to convert pdf docs to images to movie files.
I first create a temp directory for copying images, and then call ffmpeg with the arguments of input (folder), output - file name

i use for each loop for accessing different directories entered in a listbox. then for each directory i call ffmpeg.exe.

now comes the problem, for each directory in listbox i create a bmp folder
then i call ffmpeg , convert the images to movie, and delete the folder. What i want is the folder to be deleted only when the process has exited, because the folder gets deleted in midway and not all the images are converted.

How do i know when the process has exited after completing the conversion?

Dim pathmviasf As String = "c:\Aio-Pdf-Batch\Movies\Asf\"
Dim pathmvitmp As String = System.IO.Path.GetDirectoryName(Application.ExecutablePath) & "\Bmp\"

For cnt As Integer = 0 To ListBoxControl1.Items.Count - 1
                If Directory.Exists(pathmvitmp) Then
                    Directory.Delete(pathmvitmp, True)
                    Directory.CreateDirectory(pathmvitmp)
                Else
                    Directory.CreateDirectory(pathmvitmp)
                End If
                Dim i As Integer
                pdffile1 = PDFFile.Open(ListBoxControl1.Items(cnt))
                For i = 0 To pdffile1.PageCount - 1
                    Dim pageImage As Bitmap = pdffile1.GetPageImage(i, 96)
                    pageImage.Save(String.Format(pathmvitmp & "page{0}.bmp", i), ImageFormat.Bmp)
                Next i
                strfil = pathmviasf & IO.Path.GetFileNameWithoutExtension(ListBoxControl1.Items(cnt)) & "\"
                If Not Directory.Exists(strfil) Then
                    Directory.CreateDirectory(strfil)
                Else
                    Directory.Delete(strfil, True)
                    Directory.CreateDirectory(strfil)
                End If
                Dim videoPath As String = strfil & IO.Path.GetFileNameWithoutExtension(ListBoxControl1.Items(cnt)) & ".asf"
                Dim ffmpeg As Process = New Process()
                ffmpeg.StartInfo.RedirectStandardOutput = True
                ffmpeg.StartInfo.UseShellExecute = False
                ffmpeg.StartInfo.Arguments = String.Format(videoPath & " " & 0.5)
                ffmpeg.StartInfo.FileName = "ffmpeg.exe"
                ffmpeg.StartInfo.WindowStyle = ProcessWindowStyle.Hidden
                ffmpeg.Start()
            Next