Perceived Performance and Form.Paint

As a follow-up to my caution about exceptions in Form.Paint(), I wanted to illustrate why this technique is so effective. Let’s say you had a form with this code:

Private IsFirstPaint As Boolean = True
Private Sub DoWork()
Cursor = Cursors.WaitCursor
StatusBar1.Text = "Loading..."
System.Threading.Thread.Sleep(2000)
For i As Integer = 0 To 99
ComboBox1.Items.Add("ComboBoxItem " & i)
System.Threading.Thread.Sleep(5)
Next
ComboBox1.SelectedIndex = 4
System.Threading.Thread.Sleep(2000)
For i As Integer = 0 To 99
ListBox1.Items.Add("ListBoxItem " & i)
System.Threading.Thread.Sleep(5)
Next
ListBox1.SelectedIndex = 4
StatusBar1.Text = "Ready."
Cursor = Cursors.Default
End Sub
Private Sub Form1_Load(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles MyBase.Load
DoWork()
End Sub
Private Sub Form1_Paint(ByVal sender As Object, _
ByVal e As System.Windows.Forms.PaintEventArgs) _
Handles MyBase.Paint
If IsFirstPaint Then
IsFirstPaint = False
Application.DoEvents()
DoWork()
End If
End Sub

So either we’re doing 5 seconds of work in Paint, or we’re doing 5 seconds of work in form Load. Here’s what it looks like when the work is done in Form.Load:

And here’s what it looks like when the work is done in Form.Paint:

The amount of time is the same in both cases, but guess which one users will tell you is “faster?” Perceived performance is more important than actual performance.

Sure, you can do a lot better job with threading, but I guarantee that’ll take a lot more work than three lines of code! That’s why I love the IsFirstPaint and DoEvents combo: maximum benefit for minimum effort.