自定义虚线边框拖拽异常修复及Form边框绘制失效问题

Let's break down your problems and fix them step by step:

1. Fixing the Missing Form Border #

The issue here is that your Form isn't part of its own Controls collection—so your loop over Me.Controls never checks for the Form itself. The line you marked was trying to draw a border using e.ClipRectangle, but that's tied to the current child control (ComboBox/Button) in the loop, which is why it didn't work for the Form.

Here's the corrected addBorders method that draws the Form's border separately:

Protected Overrides Sub OnPaint(ByVal e As PaintEventArgs)
    addBorders(e)
End Sub

Public Sub addBorders(ByVal e As PaintEventArgs)
    ' Draw borders for child controls (TextBox, ComboBox, Button)
    For Each ctl As Control In Me.Controls
        Select Case True
            Case TypeOf ctl Is TextBox
                ctl.AutoSize = False
                ctl.Height = 19
                Dim borderRectangle As Rectangle = New Rectangle(ctl.Location, ctl.Size)
                borderRectangle.Inflate(1, 1)
                ControlPaint.DrawBorder(e.Graphics, borderRectangle, ctl.ForeColor, ButtonBorderStyle.Dashed)
            Case TypeOf ctl Is ComboBox, TypeOf ctl Is Button
                Dim borderRectangle As Rectangle = New Rectangle(ctl.Location, ctl.Size)
                borderRectangle.Inflate(1, 1)
                ControlPaint.DrawBorder(e.Graphics, borderRectangle, ctl.ForeColor, ButtonBorderStyle.Dashed)
        End Select
    Next

    ' Draw the Form's border directly (outside the child control loop)
    ' Adjust the rectangle to fit within the Form's client area
    Dim formBorderRect As Rectangle = New Rectangle(0, 0, Me.Width - 1, Me.Height - 1)
    ControlPaint.DrawBorder(e.Graphics, formBorderRect, Me.ForeColor, ButtonBorderStyle.Dashed)
End Sub

2. Fixing Border Distortion & Flickering on Screen Re-entry #

Using Me.Reload() is overkill, and a timer causes constant flickering because it refreshes too often. Instead, we'll use your IsOnScreen function to trigger a refresh only when the Form fully re-enters the screen, plus enable double-buffering to eliminate flicker.

Step 1: Enable Double-Buffering #

Add this code to your Form's Load event (or constructor) to smooth out drawing:

Private Sub YourForm_Load(sender As Object, e As EventArgs) Handles MyBase.Load
    Me.DoubleBuffered = True
    ' Set additional styles to reduce flicker
    Me.SetStyle(ControlStyles.OptimizedDoubleBuffer Or _
                ControlStyles.AllPaintingInWmPaint Or _
                ControlStyles.UserPaint, True)
    Me.UpdateStyles()
End Sub

Step 2: Track Screen State & Refresh Only When Needed #

Add a private variable to track if the Form was previously off-screen, then use the Move event to check for re-entry:

Private _wasOffScreen As Boolean = False

Private Sub YourForm_Move(sender As Object, e As EventArgs) Handles MyBase.Move
    Dim isCurrentlyOnScreen As Boolean = IsOnScreen(Me)

    ' Only refresh if we just transitioned from off-screen to fully on-screen
    If _wasOffScreen AndAlso isCurrentlyOnScreen Then
        ' Invalidate marks the Form for redraw (much smoother than Reload)
        Me.Invalidate()
    End If

    ' Update the state for the next check
    _wasOffScreen = Not isCurrentlyOnScreen
End Sub

' Keep your existing IsOnScreen function
Public Function IsOnScreen(ByVal form As Form) As Boolean
    Dim screens() As Screen = Screen.AllScreens
    For Each scrn As Screen In screens
        Dim formRectangle As Rectangle = New Rectangle(form.Left, form.Top, form.Width, form.Height)
        If scrn.WorkingArea.Contains(formRectangle) Then
            Return True
        End If
    Next
    Return False
End Function

Why This Works: #

• Double-buffering renders all drawing to an off-screen buffer first, then displays it at once—no more flicker.
• The Move event checks the Form's position only when it's moved, so we don't waste resources with constant refreshes.
• Invalidate() tells the system to redraw the Form, which is far more efficient than reloading the entire form.

内容的提问来源于stack exchange，提问作者JoeBobJr