# Appending to a File

To add to the end of a file instead of replacing it, open a StreamWriter with append set to true, or use File.AppendAllText. Either creates the file if it isn't there yet.

Journeyman · C# · Updated November 3, 2026

## After this lesson you can

- Add lines to the end of a file with new StreamWriter(path, append: true) or File.AppendAllText, keeping what's already there

## Key takeaways

- `new StreamWriter(path, append: true)` adds to the end of a file and keeps everything already there.
- `File.AppendAllText` adds no line break of its own, so include `Environment.NewLine`; `File.AppendAllLines` puts each string on its own line.
- Appending creates the file if it doesn't exist yet, so the very first append works too.
- Replace the file to save the whole list again; append to add one new record or one more line to a log.

A new student joins the class. You don't want to rewrite the whole file to add one line; you want to **append** it, adding to the end and keeping everything already there.

## append: true

`StreamWriter` has a second parameter, **`append`**. Pass `true`, and the writer starts at the end of the file instead of replacing it:

```csharp
File.WriteAllLines("student_grades.csv", new[] { "firstname,lastname,gpa", "Emily,Wilson,3.91" });

using (StreamWriter writer = new StreamWriter("student_grades.csv", append: true))
{
    writer.WriteLine("Zoe,Whitehouse,3.95");
}

Console.WriteLine(File.ReadAllText("student_grades.csv"));
```

```text
firstname,lastname,gpa
Emily,Wilson,3.91
Zoe,Whitehouse,3.95
```

Writing `append: true` with the parameter's name isn't required (`new StreamWriter(path, true)` does the same), but it tells every reader what that `true` means.

If the file doesn't exist yet, appending creates it, so the very first append works too.

## The shortcuts

`File` has append versions of its one-line writers:

| Method | Adds to the end of the file |
|---|---|
| `File.AppendAllText(path, text)` | One string, as it is: include `Environment.NewLine` or `\n` if you want a line break |
| `File.AppendAllLines(path, lines)` | Each string in a list or array, each on its own line |

```csharp
File.WriteAllText("visits.txt", "");

File.AppendAllText("visits.txt", "Monday: 40 visitors" + Environment.NewLine);
File.AppendAllText("visits.txt", "Tuesday: 55 visitors" + Environment.NewLine);
File.AppendAllLines("visits.txt", new[] { "Wednesday: 38 visitors", "Thursday: 61 visitors" });

Console.WriteLine(File.ReadAllText("visits.txt"));
```

```text
Monday: 40 visitors
Tuesday: 55 visitors
Wednesday: 38 visitors
Thursday: 61 visitors
```

## Replace or append?

| You want to | Use |
|---|---|
| Save the whole list again, such as after editing a student | `new StreamWriter(path)` or `File.WriteAllLines`: replace |
| Add one new record, or one more line to a log | `new StreamWriter(path, append: true)` or `File.AppendAllText`: append |

Appending is exactly what a log file needs: every event is one more line at the end, and nothing written before is ever lost. That's the next topic.

## Check yourself

1 question to try on the lesson page.

## So what

`append: true` turns "replace the file" into "add to the end". It's one small parameter, and choosing the wrong one either loses data or duplicates it, so choose on purpose.

## Key terms

- [Append](https://www.ask-codey.com/glossary/append-c-sharp)
- [StreamWriter](https://www.ask-codey.com/glossary/streamwriter)

## Cheat sheets

- [Appending to a File](https://www.ask-codey.com/cheat-sheets/writing-to-files-in-csharp-appending-to-a-file-cheat-sheet)

Source: https://www.ask-codey.com/lessons/writing-to-files-in-csharp-appending-to-a-file