Saturday, October 8, 2016

WinForms RichTextBox Append Log and UIThread extensions

Turn any RichTextBox in winforms to a log view. Just call rtxt.AppendLog("someString"); and it will append that to a new line in the richtextbox. And scroll it to view.

public static void AppendLog(this RichTextBox rtxt, string s)
 rtxt.UIThread(() =>
  rtxt.SelectionLength = 0;
  if (rtxt.Lines.Length > 10000)
   rtxt.Lines = rtxt.Lines.Skip(5000).ToArray();
  rtxt.SelectionStart = rtxt.TextLength;
The above snippet of code requires the following to function correctly.
static public void UIThread(this Control control, Action code)
 if (control.InvokeRequired)
I can't take credit, found it years ago on some forum somewhere. But here it is anyway. It makes a call to a winforms control synschronize with the GUI thread so that no cross-threading exceptions is thrown. GUI items can only be modified from the GUI thread and this ensures just that.

The exception thrown if correct thread synchronization is not used is: 
System.InvalidOperationException was unhandled
  Message=Cross-thread operation not valid: Control 'rtxtOutput' accessed from a thread other than the thread it was created on.
       at System.Windows.Forms.Control.get_Handle()
       at System.Windows.Forms.TextBoxBase.GetSelectionStartAndLength(Int32& start, Int32& length)
       at System.Windows.Forms.TextBoxBase.set_SelectionLength(Int32 value)
       at System.Windows.Forms.RichTextBox.set_SelectionLength(Int32 value)
       at sender, EventArgs e) in C:\tfs\YYY\Form1.cs:line 54
       at s) in C:\tfs\YYY\Form1.cs:line 38

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