C＃WinForms System.InvalidCastException

Question

私はツールストリップボタンと"frmMultiCategoryProject"と"frmTiebreaker"という2つのMDI ChildFormsを持つC＃WinFormsアプリケーションを持っています。ツールストリップボタンのTextプロパティに応じて、ツールストリップボタンのClick()イベントで異なるアクションが発生します。ここで

はコードです：

private void tsbtnAddShowTiebreaker_Click(object sender, EventArgs e) 
    { 
     if (tsbtnAddShowTiebreaker.Text == "Add Tiebreaker" || tsbtnAddShowTiebreaker.Text == "Show Tiebreaker") 
     { 
      foreach (frmMultiCategoryProject frmamdic in this.MdiChildren) 
      { 
       frmamdic.SendToBack(); 
       frmamdic.Hide(); 
       frmamdic.Visible = false; 
      } 
      frmTiebreaker frmTB = new frmTiebreaker(Tiebreaker); 
      frmTB.MdiParent = this; 
      frmTB.FormBorderStyle = System.Windows.Forms.FormBorderStyle.None; 
      frmTB.Dock = DockStyle.Fill; 
      frmTB.Show(); 
      tsbtnCloseProject.Visible = true; 
      frmTB.BringToFront(); 
      tsbtnAddShowTiebreaker.Text = "Hide Tiebreaker"; 
     } 
     else if (tsbtnAddShowTiebreaker.Text == "Hide Tiebreaker") 
     { 
      string strMdiChildren = ""; 

      foreach (frmTiebreaker frmTB in this.MdiChildren) // This is where the InvalidCastException occurs. 
      { 
       strMdiChildren += frmTB.Name.ToString() + "\r\n"; 
       this.Tiebreaker = frmTB.Tiebreaker; 
       frmTB.Dispose(); 
      } 

      foreach (frmMultiCategoryProject frmamdic in this.MdiChildren) 
      { 
       frmamdic.BringToFront(); 
       frmamdic.Show(); 
      } 

      tsbtnAddShowTiebreaker.Text = "Show Tiebreaker"; 
     } 
    } 

そして、ここで問題です（frmMain.csを：ライン2359は、コメントを有するものである）：

System.InvalidCastException was unhandled 
    HResult=-2147467262 
Message=Unable to cast object of type 'Trivia_Author_v20.frmMultiCategoryProject' to type 'Trivia_Author_v20.frmTiebreaker'. 
    Source=Trivia Author v20 
    StackTrace: 
     at Trivia_Author_v20.frmMain.tsbtnAddShowTBIsClicked() in c:\Users\Reuben\Documents\Visual Studio 2013\TRIVIA AUTHOR SUITE V20 PROJECTS 2\TAS v20 frmTB 2\Trivia Author v10 New Approach\frmMain.cs:line 2359 
     at Trivia_Author_v20.frmMain.tsbtnAddShowTiebreaker_Click(Object sender, EventArgs e) in c:\Users\Reuben\Documents\Visual Studio 2013\TRIVIA AUTHOR SUITE V20 PROJECTS 2\TAS v20 frmTB 2\Trivia Author v10 New Approach\frmMain.cs:line 2256 
     at System.Windows.Forms.ToolStripItem.RaiseEvent(Object key, EventArgs e) 
     at System.Windows.Forms.ToolStripButton.OnClick(EventArgs e) 
     at System.Windows.Forms.ToolStripItem.HandleClick(EventArgs e) 
     at System.Windows.Forms.ToolStripItem.HandleMouseUp(MouseEventArgs e) 
     at System.Windows.Forms.ToolStripItem.FireEventInteractive(EventArgs e, ToolStripItemEventType met) 
     at System.Windows.Forms.ToolStripItem.FireEvent(EventArgs e, ToolStripItemEventType met) 
     at System.Windows.Forms.ToolStrip.OnMouseUp(MouseEventArgs mea) 
     at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks) 
     at System.Windows.Forms.Control.WndProc(Message& m) 
     at System.Windows.Forms.ScrollableControl.WndProc(Message& m) 
     at System.Windows.Forms.ToolStrip.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) 
     at Trivia_Author_v20.Program.Main(String[] args) in c:\Users\Reuben\Documents\Visual Studio 2013\TRIVIA AUTHOR SUITE V20 PROJECTS 2\TAS v20 frmTB 2\Trivia Author v10 New Approach\Program.cs:line 116 
     at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args) 
     at System.AppDomain.nExecuteAssembly(RuntimeAssembly assembly, String[] args) 
     at System.Runtime.Hosting.ManifestRunner.Run(Boolean checkAptModel) 
     at System.Runtime.Hosting.ManifestRunner.ExecuteAsAssembly() 
     at System.Runtime.Hosting.ApplicationActivator.CreateInstance(ActivationContext activationContext, String[] activationCustomData) 
     at System.Runtime.Hosting.ApplicationActivator.CreateInstance(ActivationContext activationContext) 
     at System.Activator.CreateInstance(ActivationContext activationContext) 
     at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssemblyDebugInZone() 
     at System.Threading.ThreadHelper.ThreadStart_Context(Object state) 
     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.Threading.ThreadHelper.ThreadStart() 
    InnerException: 

はなぜ私のコードを伝えることができないタイプそれはいつでも対応している形式ですか？

Answer 1

foreachループでは、宣言した型にループしません。それはただキャストしようとし、失敗したときにその例外をスローします。

フィルタリングするには、.OfType<T>()を使用します。