Using the Parallel.ForEach to Speedup the execution of Loops

Question

Can the Parallel.ForEach Loop be used on values other than DataTables?
I attempted the following, but the program execution is hanging

Private Sub PictureBox1_Paint(sender As Object, e As PaintEventArgs) Handles PictureBox1.Paint
        Parallel.ForEach(Segments.OfType(Of Segment),
                          Sub(segment)
                              Try
                                   segment.DrawLine(e.Graphics)
                              Catch ex As Exception
                              End Try
                          End Sub
                          )
   End Sub

Answer 1

Hi,
If you use e.Graphics inside segment.Drawline you can get race conditions because it's impossible to use graphics in parallel. To prevent race conditions use synlock like in following demo:

Public Class Form32

  Private WithEvents PictureBoxRadarScope As New PictureBox With {.Dock = DockStyle.Fill}
  Private Segments As New List(Of Segment)
  Private Sub Form32_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    Me.Controls.Add(PictureBoxRadarScope)
    Dim rnd As New Random
    For i = 1 To 1000
      Segments.Add(New Segment With {.ID = i, .P1 = New Point(rnd.Next(10, 1000), rnd.Next(10, 1000)), .P2 = New Point(rnd.Next(10, 1000), rnd.Next(10, 1000))})
    Next
  End Sub

  Private Sub PictureBoxRadarScope_Paint(sender As Object, e As PaintEventArgs) Handles PictureBoxRadarScope.Paint
    Parallel.ForEach(Segments.OfType(Of Segment),
                  Sub(segment)
                    Try
                      segment.DrawLine(e.Graphics)
                    Catch ex As Exception
                      Debug.Print($"{segment.ID} - {ex.Message}")
                    End Try
                  End Sub
                  )
  End Sub

  Public Class Segment
    Public Sub DrawLine(gr As Graphics)
      SyncLock gr
        gr.DrawLine(Pens.Black, P1, P2)
      End SyncLock
    End Sub
    Public Property ID As Integer
    Public Property P1 As Point
    Public Property P2 As Point
  End Class

End Class