Question

我正在使用vb.net中的无边框表单，我正在尝试使用面板控件来处理表单事件。（比如mouseup，mousedown等）

我创建了一个Draggable类来处理这个问题，我尝试使用带有表单对象的类，它完美地拖动了表单。但是当我在构造函数中传递面板对象时，我得到了它。

NullReferenceException was unhandled by the user code

我可以知道为什么我总是得到nullreference异常吗？我试过实例化面板但它仍然不起作用。

更新

我在Mark Hall建议的代码中添加了Option Strict On。它显示了对象和面板的隐式转换，因此我必须将TargetObject参数设置为Panel

这是更新后的代码。它仍然提供相同的例外。我想知道一旦我在课堂上传递了面板，是否会有MouseDown, MouseUp, MouseMove个事件声明？

Draggable.vb

Option Strict On

Public Class Draggable
    Dim drag As Boolean
    Dim mousex As Integer
    Dim mousey As Integer
    Dim ctrlPanel As New Panel()

    Public Sub New(ByVal TargetObject As Panel)
        ctrlPanel = Nothing
        ctrlPanel = DirectCast(TargetObject, Panel)
        Try
            AddHandler ctrlPanel.MouseDown, New MouseEventHandler(AddressOf ThisForm_MouseDown)
            AddHandler ctrlPanel.MouseMove, New MouseEventHandler(AddressOf ThisForm_MouseMove)
            AddHandler ctrlPanel.MouseUp, New MouseEventHandler(AddressOf ThisForm_MouseUp)
        Catch ex As Exception
            MsgBox(ex.Message)
        End Try
    End Sub

    Private Sub ThisForm_MouseDown(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs)
        drag = True
        mousex = Windows.Forms.Cursor.Position.X - ctrlPanel.Left
        mousey = Windows.Forms.Cursor.Position.Y - ctrlPanel.Top
    End Sub

    Private Sub ThisForm_MouseMove(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs)
        If drag Then
            ctrlPanel.Top = Windows.Forms.Cursor.Position.Y - mousey
            ctrlPanel.Left = Windows.Forms.Cursor.Position.X - mousex
        End If
    End Sub

    Private Sub ThisForm_MouseUp(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs)
        drag = False
    End Sub
End Class

Main.vb

Public Class Main
    'Dim draggablePanel As New Draggable(ControlPanel)

#Region "Draggable Form"
    Dim drag As Boolean
    Dim mousex As Integer
    Dim mousey As Integer
    Dim ctrlPanel As Panel

    Private Sub ControlPanel_MouseDown(sender As Object, e As MouseEventArgs) Handles ControlPanel.MouseDown
        drag = True
        mousex = Windows.Forms.Cursor.Position.X - Me.Left
        mousey = Windows.Forms.Cursor.Position.Y - Me.Top
    End Sub

    Private Sub ControlPanel_MouseMove(sender As Object, e As MouseEventArgs) Handles ControlPanel.MouseMove
        If drag Then
            Me.Top = Windows.Forms.Cursor.Position.Y - mousey
            Me.Left = Windows.Forms.Cursor.Position.X - mousex
        End If
    End Sub

    Private Sub ControlPanel_MouseUp(sender As Object, e As MouseEventArgs) Handles ControlPanel.MouseUp
        drag = False
    End Sub
#End Region

    Private Sub btnMaximize_Click(sender As Object, e As EventArgs) Handles btnMaximize.Click
        If Me.WindowState = FormWindowState.Maximized Then
            Me.WindowState = FormWindowState.Normal
        Else
            Me.WindowState = FormWindowState.Maximized
        End If
    End Sub

    Private Sub btnMinimize_Click(sender As Object, e As EventArgs) Handles btnMinimize.Click
        Me.WindowState = FormWindowState.Minimized
    End Sub

    Private Sub btnClose_Click(sender As Object, e As EventArgs) Handles btnClose.Click
        Me.Close()
    End Sub

    Private Sub btnMenu_Click(sender As Object, e As EventArgs) Handles btnMenu.Click
        If pnlNav.Visible = False Then
            pnlNav.Show()
        Else
            pnlNav.Hide()
        End If
    End Sub

    Private Sub btnUser_Click(sender As Object, e As EventArgs) Handles btnUser.Click
        pnlWelcomeScreen.Hide()
        pnlSecurity.Show()
    End Sub

    Private Sub btnOptions_Click(sender As Object, e As EventArgs) Handles btnOptions.Click
        If pnlBottomOptions.Visible = False Then
            pnlBottomOptions.Show()
        Else
            pnlBottomOptions.Hide()
        End If
    End Sub
End Class

异常来自

Public Sub New(ByVal TargetObject As Panel)
    ctrlPanel = Nothing
    ctrlPanel = DirectCast(TargetObject, Panel)
    Try
        this one here -> AddHandler ctrlPanel.MouseDown, New MouseEventHandler(AddressOf ThisForm_MouseDown) 'this one here
        AddHandler ctrlPanel.MouseMove, New MouseEventHandler(AddressOf ThisForm_MouseMove)
        AddHandler ctrlPanel.MouseUp, New MouseEventHandler(AddressOf ThisForm_MouseUp)
    Catch ex As Exception
        MsgBox(ex.Message)
    End Try
End Sub

表格的屏幕截图

https://drive.google.com/file/d/0B1AyzK5zBH9ZNUN0eF9YcGRna2M/edit?usp=sharing

感谢您的所有意见和帮助..

Answer 1

Sir ChrisDunaway建议在Form_Load中声明该类，而不是在Class中声明它。

<强> Draggable.vb

Public Class Draggable

    Dim drag As Boolean
    Dim mousex As Integer
    Dim mousey As Integer
    Dim ctrlPanel As Panel
    Dim ctrlForm As Form
    Public Sub New(ByVal TargetObject As Panel, ByVal TargetForm As Form)
        ctrlPanel = TargetObject
        ctrlForm = TargetForm
        Try
            AddHandler ctrlPanel.MouseDown, New MouseEventHandler(AddressOf ThisForm_MouseDown)
            AddHandler ctrlPanel.MouseMove, New MouseEventHandler(AddressOf ThisForm_MouseMove)
            AddHandler ctrlPanel.MouseUp, New MouseEventHandler(AddressOf ThisForm_MouseUp)
        Catch ex As Exception
            MsgBox(ex.Message)
        End Try
End Sub

<强> Main.vb

Public Class Main

    Private Sub Main_Load(sender As Object, e As EventArgs) Handles Me.Load
        Dim draggablePanel As New Draggable(ControlPanel, Me)
    End Sub

<强>更新

我尝试添加Public Sub New()并在InitializeComponent()之后编写代码，但我仍然获得相同的效果。：d

    Public Sub New()
    InitializeComponent()
    Dim controlBar As New ControlBar(Me, pnlControlBar, btnClose)
End Sub

非常感谢你的建议......

PanelControl在传递给类构造函数时会获得NullReferenceException

1 个答案: