Write data to files

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Writing to a file lets your program save results, keep logs, or persist data between runs. C# offers a few ways to put data into a file, split mainly between overwriting and appending.

Writing to a file with File.WriteAllText()

File.WriteAllText() creates a new file, or completely overwrites an existing file with the same name.

File.WriteAllText("greeting.txt", "Hello, world!\n");

Warning

File.WriteAllText() erases the file's existing content the moment it's called. Only use it when you're sure you want to replace the file's contents.

Appending to a file with File.AppendAllText()

File.AppendAllText() adds to the end of the file, keeping anything that's already there. If the file doesn't exist yet, C# creates it.

File.AppendAllText("log.txt", "New entry added.\n");

Appending is a good fit for log files, notes, and any data you want to build up over time.

Writing multiple lines with StreamWriter

For more control, like writing many lines without building one giant string first, use a StreamWriter with using. Its constructor takes an optional second parameter that sets append mode.

string[] tasks = { "Buy groceries", "Walk the dog", "Finish homework" };

using StreamWriter writer = new StreamWriter("tasks.txt", append: false);

foreach (string task in tasks)
{
    writer.WriteLine(task);
}

.WriteLine() adds a newline after each entry automatically, unlike .Write().

Choosing between write and append

Use write when...	Use append when...
You want to start fresh each time	You want to keep the existing content
You're saving the final result of a process	You're recording new events or entries
Overwriting is intentional	Preserving history matters