Question

我尝试调试

时收到以下异常

An unhandled exception of type 'System.NullReferenceException' occurred in HomeWork Helper.exe

Additional information: Object reference not set to an instance of an object.

它来自AddControl表单加载事件

中的这行代码

lblDescription.Text = "This Window is designed to set reminders for you for " & TypeAssign & "."

以下是模块宣言

Public TypeAssign As String

在表单打开之前已在此处指定变量TypeAssign

TypeAssign = "Tests"
frmControls.ShowDialog()

Answer 1

如果您在表单构造函数中编写该代码，请记住在调用InitializeComponent之后将其放入，否则将声明控件lblDescription但它尚未初始化，因此使用Text属性会导致NullReferenceException。 / p>

Public Class AddControl

     Public Sub New()
         InitializeComponent()
         lblDescription.Text = "This Window is designed to set reminders for you for " & _
                                TypeAssign 
     End Sub

当然，如果在AddControl类中看不到TypeAssign，这将不起作用。在这种情况下，简单的解决方案是通过新版本的Form构造函数

传递变量

Public Class AddControl

     Public Sub New(typeAssign as string)
         InitializeComponent()
         lblDescription.Text = "This Window is designed to set reminders for you for " & _
                                typeAssign 
     End Sub

通过

调用第二个版本

TypeAssign = "Tests"
AddControl frmControls = new AddControl(TypeAssign)
frmControls.ShowDialog()
frmControls.ShowDialog()

尝试分配标签文本时出现NullReferenceException

1 个答案: