How to Write to a Text File in C#

#file-io #fundamentals

Writing to a file in C# is straightforward with the static File class. For simple cases, File.WriteAllText and File.WriteAllLines cover most needs. For streaming large amounts of data or appending incrementally, use StreamWriter.

File.WriteAllText — Write a Single String

Creates the file if it doesn't exist, or overwrites it if it does.

C# Example Code

using System.IO;

string path = "notes.txt";

File.WriteAllText(path, "Hello, World!");

// With a specific encoding
using System.Text;
File.WriteAllText(path, "Héllo, Wörld!", Encoding.UTF8);

File.WriteAllLines — Write a Collection of Lines

Writes each element of the collection on its own line.

C# Example Code

string[] lines = { "Line one", "Line two", "Line three" };
File.WriteAllLines("output.txt", lines);

// Works with any IEnumerable<string>
List<string> names = new() { "Alice", "Bob", "Carol" };
File.WriteAllLines("names.txt", names);

File.AppendAllText — Append Without Overwriting

C# Example Code

string logPath = "log.txt";

File.AppendAllText(logPath, $"[{DateTime.Now}] Application started{Environment.NewLine}");
File.AppendAllText(logPath, $"[{DateTime.Now}] Processing complete{Environment.NewLine}");

File.AppendAllLines — Append Multiple Lines

C# Example Code

string[] newEntries = { "Entry 4", "Entry 5" };
File.AppendAllLines("log.txt", newEntries);

StreamWriter — Full Control Over Writing

Use StreamWriter when you need to write incrementally, control buffering, or keep the file open across multiple writes.

C# Example Code

using StreamWriter writer = new StreamWriter("report.txt");

writer.WriteLine("=== Report ===");
writer.WriteLine($"Generated: {DateTime.Now:yyyy-MM-dd HH:mm}");
writer.WriteLine();

for (int i = 1; i <= 5; i++)
    writer.WriteLine($"Item {i}: value {i * 10}");

// writer.Dispose() is called automatically by the using statement

Append to an existing file by passing append: true:

C# Example Code

using StreamWriter writer = new StreamWriter("log.txt", append: true);
writer.WriteLine($"[{DateTime.Now:HH:mm:ss}] New log entry");

Async Variants — WriteAllTextAsync and WriteAllLinesAsync

For I/O-bound operations in async methods, prefer the async overloads to avoid blocking threads.

C# Example Code

using System.IO;

await File.WriteAllTextAsync("async-output.txt", "Written asynchronously.");

string[] lines = { "Alpha", "Beta", "Gamma" };
await File.WriteAllLinesAsync("async-lines.txt", lines);

// StreamWriter also supports async
await using StreamWriter writer = new StreamWriter("async-log.txt");
await writer.WriteLineAsync("Async line written.");

Writing JSON or Structured Data

For structured data, serialize first and then write.

C# Example Code

using System.Text.Json;

var config = new { Host = "localhost", Port = 5432, Debug = true };
string json = JsonSerializer.Serialize(config, new JsonSerializerOptions { WriteIndented = true });

await File.WriteAllTextAsync("config.json", json);

Error Handling

Wrap file operations in try/catch to handle missing directories, permission errors, or disk-full conditions.

C# Example Code

string path = @"C:\Logs\app.log";

try
{
    // Ensure the directory exists before writing
    Directory.CreateDirectory(Path.GetDirectoryName(path)!);
    await File.AppendAllTextAsync(path, $"[{DateTime.Now}] Entry written{Environment.NewLine}");
}
catch (UnauthorizedAccessException ex)
{
    Console.Error.WriteLine($"Permission denied: {ex.Message}");
}
catch (IOException ex)
{
    Console.Error.WriteLine($"I/O error: {ex.Message}");
}

Real-World Example: CSV Export

C# Example Code

public record Product(string Name, decimal Price, int Stock);

List<Product> products = new()
{
    new("Widget",  9.99m,  120),
    new("Gadget",  24.95m, 45),
    new("Doohickey", 4.50m, 300),
};

var lines = new List<string> { "Name,Price,Stock" };
lines.AddRange(products.Select(p => $"{p.Name},{p.Price:F2},{p.Stock}"));

await File.WriteAllLinesAsync("products.csv", lines);

// products.csv:
// Name,Price,Stock
// Widget,9.99,120
// Gadget,24.95,45
// Doohickey,4.50,300

Quick Reference

Scenario	Method
Write a string (overwrite)	`File.WriteAllText(path, text)`
Write multiple lines (overwrite)	`File.WriteAllLines(path, lines)`
Append a string	`File.AppendAllText(path, text)`
Append multiple lines	`File.AppendAllLines(path, lines)`
Incremental / streaming write	`new StreamWriter(path)`
Append with StreamWriter	`new StreamWriter(path, append: true)`
Async write	`File.WriteAllTextAsync(path, text)`
Ensure directory exists	`Directory.CreateDirectory(dir)`

How to Write to a Text File in C#

File.WriteAllText — Write a Single String

File.WriteAllLines — Write a Collection of Lines

File.AppendAllText — Append Without Overwriting

File.AppendAllLines — Append Multiple Lines

StreamWriter — Full Control Over Writing

Async Variants — WriteAllTextAsync and WriteAllLinesAsync

Writing JSON or Structured Data

Error Handling

Real-World Example: CSV Export

Quick Reference

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