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I have a richTextBox I am using to perform some syntax highlighting. This is a small editing facility so I have not written a custom syntax highlighter - instead I am using Regex s and updating upon the detection of an input delay using an event handler for the Application.Idle event: Application.Idle += new EventHandler(Application_Idle); in the event handler I check for the time the text box has been inactive: private void Application_Idle(object sender, EventArgs e) { // Get time since last syntax update. double timeRtb1 = DateTime.Now.Subtract(_lastChangeRtb1).TotalMilliseconds; // If required highlight syntax. if (timeRtb1 > MINIMUM_UPDATE_DELAY) { HighlightSyntax(ref richTextBox1); _lastChangeRtb1 = DateTime.MaxValue; } } But even for relatively small highlights the RichTextBox flickers heavily and it has no richTextBox.BeginUpdate()/EndUpdate() methods. To overcome this I found this answer to a similar dilemma by Hans Passant (Hans Passant has never let me down!): using System; using System.Windows.Forms; using System.Runtime.InteropServices; class MyRichTextBox : RichTextBox { public void BeginUpdate() { SendMessage(this.Handle, WM_SETREDRAW, (IntPtr)0, IntPtr.Zero); } public void EndUpdate() { SendMessage(this.Handle, WM_SETREDRAW, (IntPtr)1, IntPtr.Zero); } [DllImport("user32.dll")] private static extern IntPtr SendMessage(IntPtr hWnd, int msg, IntPtr wp, IntPtr lp); private const int WM_SETREDRAW = 0x0b; } However, this gives me odd behaviour upon an update; the cursor dies/freezes and shows nothing but odd looking stripes (see image below). <img src="https://i.sstatic.net/5ni0R.jpg" alt="Odd Error Caused by RichTextBox Method Ext

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