public class MyForm : Form

{
    // ..

    private const int WM_NCHITTEST = 0x0084; 
    private const int HTCLIENT = 1;
    private const int HTCAPTION = 2;

    protected override void WndProc( ref Message m ) 
    {
        base.WndProc( ref m );

        
        if ( m.Msg == WM_NCHITTEST ) 
        {
            if ( m.Result == ( IntPtr )HTCLIENT ) 
            {
                m.Result = ( IntPtr )HTCAPTION; 
            }
        }
    }
}

Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" _ 
                 (ByVal hwnd As Long, ByVal wMsg As Long, _ 
                  ByVal wParam As Long, lParam As Any) As Long 
 Private Declare Sub ReleaseCapture Lib "user32" () 
 
 Private Const WM_NCLBUTTONDOWN = &HA1 
 Private Const HTCAPTION = 2 
 
 Private Sub Form_MouseMove(Button As Integer, Shift As Integer, X As Single, Y As Single) 
     If Button = vbLeftButton Then
        ReleaseCapture
        SendMessage Me.hwnd, WM_NCLBUTTONDOWN, HTCAPTION, 0&
    End If
 End Sub

Private Declare Function ReleaseCapture Lib "user32" () As Integer
Private Const HTCAPTION As Integer = 2
Private Const WM_NCLBUTTONDOWN As Integer = &HA1S
Private Declare Function SendMessage Lib "user32" Alias "SendMessageA" (ByVal hwnd As Integer, ByVal wMsg As Integer, ByVal wParam As Integer, ByRef lParam As Integer) As Integer
Private Sub Form_MouseDown(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles Me.MouseDown
    If e.Button = Windows.Forms.MouseButtons.Left Then
        ReleaseCapture()
        SendMessage(Me.Handle.ToInt32, WM_NCLBUTTONDOWN, HTCAPTION, 0)
    End If
End Sub

procedure TForm1.FormMouseMove(Sender: TObject; Shift: TShiftState; X,
  Y: Integer);
begin
  ReleaseCapture;
  Perform (WM_SYSCOMMAND, SC_MOVE or HTCAPTION, 0);
end;

Private Const WM_NCHITTEST As Integer = &H84
Private Const HTCLIENT As Integer = &H1
Private Const HTCAPTION As Integer = &H2

Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)
  MyBase.WndProc(m)
  If m.Msg = WM_NCHITTEST Then
    If m.Result = CType(HTCLIENT, IntPtr) Then
      m.Result = CType(HTCAPTION, IntPtr)
    End If
  End If
End Sub