read data from text box to text file continuousl when enter

Well, you can definitely go with File.AppendAllText(FilePath,Content, NewLineChar) method.

 

In order to do that, you can place this method call in TextBox's Textchanged event and you should be good.

 

But, I would suggest to create some sort of delay while writing to the file because while use is editing the text ( and your method call writing text to the file), UI may be little freezy. To get rid of that, you can either make a delayed call using timer or you can do that on seperate thread/task.