C# WinForms System.InvalidCastException

Question

Ich habe eine C# WinForms App mit einem Toolstrip-Button und zwei MDI ChildForms namens "frmMultiCategoryProject" und "frmTiebreaker". Im Ereignis Click() der Tooltrip-Schaltfläche treten je nach der Eigenschaft Text der Tooltrip-Schaltfläche unterschiedliche Aktionen auf.

Hier ist der Code:

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"; 
     } 
    } 

Und hier ist das Problem (frmMain.cs: Linie 2359 der mit dem Kommentar ist):

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: 

Warum kann mein Code nicht sagen, welche Art von der Form, mit der es sich in jedem Moment beschäftigt?

Answer 1

Die Schleife foreach filtert ihre Elemente nicht zu dem Typ, den Sie deklarieren; es versucht nur zu werfen und wird diese Ausnahme auslösen, wenn es fehlschlägt.

Zum Filtern verwenden Sie .OfType<T>().