AccessVoilationException with richtextbox

I am using VS 2015 with .Net framework 5.2.1 and winforms. The rich text box calls are making the application crash intermittently

with System.AccessViolationException:

At the bottom there is error from Event Viewer and it seems it is crashing while setting Selection start. I set Selection start at two

places in my code. The crash is intermittent. I will really appreciate any pointers.

1) While scrolling it to the bottom:

richTextBox1.SelectionStart = richTextBox1.Text.Length;

richTextBox1.ScrollToCaret();

2) while appending text to the richtextbox:

public static void AppendText(this RichTextBox box, string text, Color color)

{

box.SelectionStart = box.TextLength;

box.SelectionLength = 0;

box.SelectionColor = color;

box.AppendText(text);

}

EventViewer:

Application: AutomationHost.exe

Framework Version: v4.0.30319

Description: The process was terminated due to an unhandled exception.

Exception Info: System.AccessViolationException

at System.Windows.Forms.UnsafeNativeMethods.CallWindowProc(IntPtr, IntPtr, Int32, IntPtr, IntPtr)

at System.Windows.Forms.NativeWindow.DefWndProc(System.Windows.Forms.Message ByRef)

at System.Windows.Forms.Control.DefWndProc(System.Windows.Forms.Message ByRef)

at System.Windows.Forms.Control.WndProc(System.Windows.Forms.Message ByRef)

at System.Windows.Forms.TextBoxBase.WndProc(System.Windows.Forms.Message ByRef)

at System.Windows.Forms.RichTextBox.WndProc(System.Windows.Forms.Message ByRef)

at System.Windows.Forms.Control+ControlNativeWindow.OnMessage(System.Windows.Forms.Message ByRef)

at System.Windows.Forms.Control+ControlNativeWindow.WndProc(System.Windows.Forms.Message ByRef)

at System.Windows.Forms.NativeWindow.Callback(IntPtr, Int32, IntPtr, IntPtr)

at System.Windows.Forms.UnsafeNativeMethods.SendMessage(System.Runtime.InteropServices.HandleRef, Int32, Int32, EDITSTREAM)

at System.Windows.Forms.RichTextBox.StreamOut(System.IO.Stream, Int32, Boolean)

at System.Windows.Forms.RichTextBox.StreamOut(Int32)

at System.Windows.Forms.RichTextBox.get_SelectedText()

at System.Windows.Forms.RichTextBox.get_SelectionLength()

at System.Windows.Forms.TextBoxBase.set_SelectionStart(Int32)

at AutomationHostHelper.Test.ThreadSafeRichTextBox()

Exception Info: System.Reflection.TargetInvocationException

at System.RuntimeMethodHandle.InvokeMethod(System.Object, System.Object[], System.Signature, Boolean)

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

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

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

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

at System.Threading.ExecutionContext.RunInternal(System.Threading.ExecutionContext, System.Threading.ContextCallback,

System.Object, Boolean)

at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object,

Boolean)

at System.Threading.ExecutionContext.Run(System.Threading.ExecutionContext, System.Threading.ContextCallback, System.Object)

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

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

at System.Windows.Forms.Control.WndProc(System.Windows.Forms.Message ByRef)

at System.Windows.Forms.TextBoxBase.WndProc(System.Windows.Forms.Message ByRef)

at System.Windows.Forms.RichTextBox.WndProc(System.Windows.Forms.Message ByRef)

at System.Windows.Forms.Control+ControlNativeWindow.OnMessage(System.Windows.Forms.Message ByRef)

at System.Windows.Forms.Control+ControlNativeWindow.WndProc(System.Windows.Forms.Message ByRef)

at System.Windows.Forms.NativeWindow.Callback(IntPtr, Int32, IntPtr, IntPtr)

at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG ByRef)

at System.Windows.Forms.Application

+ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr, Int32, Int32)

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

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

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

at AutomationHost.Program.Main(System.String[])