Check out example codes for "c# append to file". It will help you in understanding the concepts better.

Code Example 1

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

Code Example 2

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

Code Example 3

using (FileStream fs = new FileStream(fileName,FileMode.Append, FileAccess.Write))
 using (StreamWriter sw = new StreamWriter(fs))
 {
    sw.WriteLine(something);
 }

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