Draw line in text file

If you mean you want to insert lines as text inside your text files, you can programmatically insert a text like "_________" as shown in the following forum:
http://stackoverflow.com/questions/16212127/add-a-new-line-at-a-specific-position-in-a-text-file

But if you mean you want to draw a formatted line (that has color, thickness, etc.), to my knowledge, this is not supported in a regular text files. If this is the case, you have two options here:
1. Draw graphic lines inside a Rich Text file (instead of regular text) as shown below:
http://www.codeproject.com/Questions/656352/draw-line-in-rich-textbox
2. Load the text file as image (rasterize the text file) using a programming toolkit like leadtools. For details, see the following link:
https://www.leadtools.com/help/leadtools/v19/dh/co/leadtools.codecs~leadtools.codecs.codecstxtoptions.html
After this, you can draw lines over it using the Graphics.DrawLine() method. Keep in mind that the result file in this case is image file, not text. So you will need to save the result image using the System.Drawing.Image.Save() method as shown here:
https://msdn.microsoft.com/en-us/library/9t4syfhh%28v=vs.110%29.aspx