keep the pre existing text in text file via C# and write to the end of it code example

Example 1: c# append to file

using(StreamWriter writer = new StreamWriter("C:\\log.txt", true)) 
{ 
	writer.WriteLine("Line"); 
}

Example 2: c# append to file

File.AppendAllText(@"c:\path\file.txt", "text content" + Environment.NewLine);