System.ObjectDisposedException when the button style is set to FlatStyle.System in a WinForms application

Question

System.ObjectDisposedException when the button style is set to FlatStyle.System in a WinForms application

I have a child form that appears when a button is clicked in the parent. And the code is as follows.

 Public Class Parent Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load End Sub Private Sub btnOpenChild_Click(sender As Object, e As EventArgs) Handles btnOpenChild.Click Child.Show() End Sub End Class 

The detailed form, in turn, has a button that closes.

 Public Class Child Private Sub btnCloseMe_Click(sender As Object, e As EventArgs) Handles btnCloseMe.Click Me.Close() End Sub End Class 

Steps to Exclude:

• In debug mode, set a breakpoint on Me.Close()
• Then click the close button of the child.
• When you hit a breakpoint, open notepad and
• Go back to the solution, then continue

An exception:

 System.ObjectDisposedException was unhandled HResult=-2146232798 Message=Cannot access a disposed object. Object name: 'Button'. Source=System.Windows.Forms ObjectName=Button StackTrace: at System.Windows.Forms.Control.CreateHandle() at System.Windows.Forms.Control.get_Handle() at System.Windows.Forms.Control.PointToScreen(Point p) at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent) at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks) at System.Windows.Forms.Control.WndProc(Message& m) at System.Windows.Forms.ButtonBase.WndProc(Message& m) at System.Windows.Forms.Button.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(ApplicationContext context) at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.OnRun() at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.DoApplicationModel() at Microsoft.VisualBasic.ApplicationServices.WindowsFormsApplicationBase.Run(String[] commandLine) at WindowsApplication2.My.MyApplication.Main(String[] Args) in 17d14f5c-a337-4978-8281-53493378c1071.vb:line 81 at System.AppDomain._nExecuteAssembly(RuntimeAssembly assembly, String[] args) at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args) at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly() 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: 

Does anyone know this clearly happens when the button style is set to FlatStyle.System

Answer 1

This is soon a mistake. The internal implementation of Button / ButtonBase supports some flags related to the state of the mouse, which in this case are not cleared properly. FlatStyle.System seems to be a special case involving many branches in the source code, so some of them seem to be missing something.

A workaround would be to create and use your own Button subclass like this (sorry for C #, I'm sure you can translate it to VB):

 public class MyButton : Button { protected override void OnMouseUp(MouseEventArgs mevent) { if (this.IsDisposed) return; base.OnMouseUp(mevent); } }