Illegal cross thread operation exception, unable to handle it

Question

Hello, i am using visual C# express 2008. I SOMETIMES get the illegal cross thread operation in my invoke method when i try to run my project. It use the same invoke method in many places but its only at the beginning of another thread that i get the error. I check the InvokeRequired property, invoke the same method and in the 'else' condition and create a temporary variable and assign it the text of my control. But on that line, in the 'else' statement, inside the Invoking method i sometimes get the exception. What could be the cause? How to get rid of it? It does not occur often, but it is still a bug...

Code:

// Delegate:
private delegate void ChangeTextDelegate(string text);

// Method:
public static void ChangeText(string text)
{
    if (richtextbox1.InvokeRequired)
    {
        richtextbox1.Invoke(new ChangeTextDelegate(ChangeText), new object[] { text });
    }
    else
    {
        int startIndex;
        startIndex = richtextbox1.TextLength; // <- Exception points here.
        // ...
    }
}

Stack trace:

System.InvalidOperationException was unhandled
  Message="Cross-thread operation not valid: Control 'SomeClass' accessed from a thread other than the thread it was created on."
  Source="System.Windows.Forms"
  StackTrace:
       at System.Windows.Forms.Control.get_Handle()
       at System.Windows.Forms.Control.get_InternalHandle()
       at System.Windows.Forms.Control.WmWindowPosChanged(Message& m)
       at System.Windows.Forms.Control.WndProc(Message& m)
       at System.Windows.Forms.TextBoxBase.WndProc(Message& m)
       at System.Windows.Forms.RichTextBox.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.CallWindowProc(IntPtr wndProc, IntPtr hWnd, Int32 msg, IntPtr wParam, IntPtr lParam)
       at System.Windows.Forms.NativeWindow.DefWndProc(Message& m)
       at System.Windows.Forms.Control.DefWndProc(Message& m)
       at System.Windows.Forms.Control.WmCreate(Message& m)
       at System.Windows.Forms.Control.WndProc(Message& m)
       at System.Windows.Forms.TextBoxBase.WndProc(Message& m)
       at System.Windows.Forms.RichTextBox.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.IntCreateWindowEx(Int32 dwExStyle, String lpszClassName, String lpszWindowName, Int32 style, Int32 x, Int32 y, Int32 width, Int32 height, HandleRef hWndParent, HandleRef hMenu, HandleRef hInst, Object pvParam)
       at System.Windows.Forms.UnsafeNativeMethods.CreateWindowEx(Int32 dwExStyle, String lpszClassName, String lpszWindowName, Int32 style, Int32 x, Int32 y, Int32 width, Int32 height, HandleRef hWndParent, HandleRef hMenu, HandleRef hInst, Object pvParam)
       at System.Windows.Forms.NativeWindow.CreateHandle(CreateParams cp)
       at System.Windows.Forms.Control.CreateHandle()
       at System.Windows.Forms.TextBoxBase.CreateHandle()
       at System.Windows.Forms.Control.get_Handle()
       at System.Windows.Forms.RichTextBox.get_TextLength()
       at SomeNamespace.SomeClass.Changetext(String text, Color color) in C:\...\SomeClass.cs:line 827
       at SomeNamespace.SomeClass.SomeThreadFun() in C:\...\SomeClass.cs:line 112
       at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
       at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
       at System.Threading.ThreadHelper.ThreadStart()
  InnerException: 

TY

Answer 1

The issue seems to be that you're trying to access a Windows control property from a non-UI thread. Instead, try to use Invoke to get the text length from the control.

Answer 2

I access the RTB though my Form instance, from another class, from another thread. And i get the error when i start debugging not when i close the app. Could it still be the same issue?