Question

有一些mdi应用程序。在该应用程序中，用户可以打开某个表单来完成一些工作。他可以打开多种形式。每次他打开一个新的，他要求在小形式之前提供IP地址，哪个值传递给表单。在每个表单中，我正在尝试实现某些引擎，以便不时给出ip，并在状态栏控件上显示它的状态。我决定使用async/await，但是我有问题，当用户输入IP地址并单击确定按钮，然后出现而不是FrmDrukujEtykiete，我的主应用程序类中出现错误。你能支持我如何使它有效吗？

这是出现错误的地方：

   Public Sub main()

        Application.EnableVisualStyles()
        Dim result As DialogResult
        Using frmL As New FrmLogin
            result = frmL.ShowDialog
        End Using

        If result = DialogResult.OK Then
            Dim FrmMain As New FrmMain()
            Application.Run(FrmMain)
        End If
    End Sub
End Module

此行显示错误：

Application.Run(FrmMain)

那句话：

Additional information: Exception has been thrown by the target of an invocation.

所以一步一步：用户点击mdi格式的按钮somwhere然后有这个代码：

Dim pobierzIP As New FrmPobierzIP  'asking form to get ip
If pobierzIP.ShowDialog() = DialogResult.OK Then
Dim drukujEtykiete As New FrmDrukujEtykiete(pobierzIP.Ip)  'targeted form
....

这就是我正在尝试为我的ping实现async / await的地方 - 这里的错误是错误的：

Public Class FrmDrukujEtykiete

    Private Property IPAddress As String

    Sub New(ipaddress As String)
        InitializeComponent()
        PingPong()
        Me.IPAddress = ipaddress
    End Sub
    Private Sub btnWyjdz_Click(sender As Object, e As EventArgs) Handles btnWyjdz.Click
        Close()
    End Sub

    Private Async Sub PingPong()
        While (True)
            Dim mytask As Task(Of Boolean)
            mytask = Task.Factory.StartNew(IsDestinationReachable(IPAddress))

            If Await mytask Then
                tsPingResultIcon.BackColor = Color.Green
                tsPingResultIcon.Text = "Reachable"
            Else
                tsPingResultIcon.BackColor = Color.Red
                tsPingResultIcon.Text = "Not reachable"
            End If

        End While
    End Sub

    Public Function IsDestinationReachable(ByVal hostnameOrAddress As String)
        Dim reachable As Boolean = False
        Try
            reachable = My.Computer.Network.IsAvailable AndAlso
                             My.Computer.Network.Ping(hostnameOrAddress)

        Catch pingException As System.Net.NetworkInformation.PingException
        Catch genericNetworkException As System.Net.NetworkInformation.NetworkInformationException

            ' Fail silently and return false
        End Try
        Return reachable
    End Function


End Class

编辑：根据建议我在Application.Run（FrmMain）中尝试捕获这是我得到的错误：

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.ArgumentNullException: Value cannot be null.

Parameter name: hostNameOrAddress

   at System.Net.NetworkInformation.Ping.Send(String hostNameOrAddress, Int32 timeout, Byte[] buffer, PingOptions options)

   at System.Net.NetworkInformation.Ping.Send(String hostNameOrAddress, Int32 timeout, Byte[] buffer)

   at Microsoft.VisualBasic.Devices.Network.Ping(String hostNameOrAddress, Int32 timeout)

   at Microsoft.VisualBasic.Devices.Network.Ping(String hostNameOrAddress)

   at FrmDrukujEtykiete.IsDestinationReachable(String hostnameOrAddress) in C:\Pliki\_Projekty\Tompson\Tompson\Tompson\FrmDrukujEtykiete.vb:line 37

   at FrmDrukujEtykiete.VB$StateMachine_70_PingPong.MoveNext() in C:\Pliki\_Projekty\Tompson\Tompson\Tompson\FrmDrukujEtykiete.vb:line 21

--- End of stack trace from previous location where exception was thrown ---

   at System.Runtime.CompilerServices.AsyncMethodBuilderCore.<>c.<ThrowAsync>b__6_0(Object state)

   --- End of inner exception stack trace ---

   at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)

   at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)

   at System.Delegate.DynamicInvokeImpl(Object[] args)

   at System.Windows.Forms.Control.InvokeMarshaledCallbackDo(ThreadMethodEntry tme)

   at System.Windows.Forms.Control.InvokeMarshaledCallbackHelper(Object obj)

   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)

   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)

   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)

   at System.Windows.Forms.Control.InvokeMarshaledCallback(ThreadMethodEntry tme)

   at System.Windows.Forms.Control.InvokeMarshaledCallbacks()

   at System.Windows.Forms.Control.WndProc(Message& m)

   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)

   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)

   at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

   at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)

   at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)

   at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)

   at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)

   at System.Windows.Forms.Application.Run(Form mainForm)

EDIT3：

System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> System.InvalidCastException: Unable to cast object of type 'System.Boolean' to type 'System.Action'.

   at FrmDrukujEtykiete.VB$StateMachine_70_PingPong.MoveNext() in C:\Pliki\_Projekty\sol\FrmDrukujEtykiete.vb:line 22

--- End of stack trace from previous location where exception was thrown ---

   at System.Runtime.CompilerServices.AsyncMethodBuilderCore.<>c.<ThrowAsync>b__6_0(Object state)

   --- End of inner exception stack trace ---

   at System.RuntimeMethodHandle.InvokeMethod(Object target, Object[] arguments, Signature sig, Boolean constructor)

   at System.Reflection.RuntimeMethodInfo.UnsafeInvokeInternal(Object obj, Object[] parameters, Object[] arguments)

   at System.Delegate.DynamicInvokeImpl(Object[] args)

   at System.Windows.Forms.Control.InvokeMarshaledCallbackDo(ThreadMethodEntry tme)

   at System.Windows.Forms.Control.InvokeMarshaledCallbackHelper(Object obj)

   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)

   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)

   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)

   at System.Windows.Forms.Control.InvokeMarshaledCallback(ThreadMethodEntry tme)

   at System.Windows.Forms.Control.InvokeMarshaledCallbacks()

   at System.Windows.Forms.Control.WndProc(Message& m)

   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)

   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)

   at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)

   at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)

   at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData)

   at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)

   at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)

   at System.Windows.Forms.Application.Run(Form mainForm)

Answer 1

错误表明调用hostNameOrAddress时System.Net.NetworkInformation.Ping.Send不能为空。

这使人们相信它没有收到传递给它的价值。

您的PingPong()方法正在使用私有属性IPAddress来了解要ping的内容。但是，构造函数在为该属性赋值之前调用PingPong() ，因此当PingPong()调用.Ping()时，IPAddress字段为空。

Sub New(ipaddress As String) InitializeComponent() PingPong() Me.IPAddress = ipaddress End Sub

交换构造函数的第二行和第三行，在调用PingPong()之前将值赋给属性。

许多表单的Ping方法为异步/等待

1 个答案: