C# RichTextBox Highlight Line

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You’ll need to create your own control that inherits from RichTextBox and use that control on your form. Since the RichTextBox does not support owner drawing, you will have to listen for the WM_PAINT message and then do your work in there. Here is an example that works fairly well, though the line height is hard coded right now:

 public class HighlightableRTB : RichTextBox
 {
     // You should probably find a way to calculate this, as each line could have a different height.
     private int LineHeight = 15; 
     public HighlightableRTB()
     {
         HighlightColor = Color.Yellow;
     }

    [Category("Custom"),
    Description("Specifies the highlight color.")]
     public Color HighlightColor { get; set; }

     protected override void OnSelectionChanged(EventArgs e)
     {
         base.OnSelectionChanged(e);
         this.Invalidate();
     }

     private const int WM_PAINT = 15;

     protected override void WndProc(ref Message m)
     {
         if (m.Msg == WM_PAINT)
         {
             var selectLength = this.SelectionLength;
             var selectStart = this.SelectionStart;

             this.Invalidate();
             base.WndProc(ref m);

             if (selectLength > 0) return;   // Hides the highlight if the user is selecting something

             using (Graphics g = Graphics.FromHwnd(this.Handle))
             {
                 Brush b = new SolidBrush(Color.FromArgb(50, HighlightColor));
                 var line = this.GetLineFromCharIndex(selectStart);
                 var loc = this.GetPositionFromCharIndex(this.GetFirstCharIndexFromLine(line));

                 g.FillRectangle(b, new Rectangle(loc, new Size(this.Width, LineHeight)));
             }
         }
         else
         {
             base.WndProc(ref m);
         }
     }
 }

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