File.ReadAllText() / File.WriteAllText()

using System.IO;

// Reads all content from a file and returns it as a string.
string content = File.ReadAllText(string path);
string content = File.ReadAllText(string path, Encoding encoding);

// Writes a string to a file (overwrites existing content).
File.WriteAllText(string path, string? contents);
File.WriteAllText(string path, string? contents, Encoding encoding);

// Reads all lines from a file and returns them as a string array (one element per line).
string[] lines = File.ReadAllLines(string path);

// Writes a string array to a file, one line at a time (overwrites existing content).
File.WriteAllLines(string path, IEnumerable<string> contents);

// Appends text to the end of a file.
File.AppendAllText(string path, string? contents);

using System;
using System.IO;
using System.Text;

string filePath = "sample.txt";

// Create a file using File.WriteAllText().
string writeContent = "Line 1: Hello, C#\nLine 2: File I/O\nLine 3: Done";
File.WriteAllText(filePath, writeContent, Encoding.UTF8);
Console.WriteLine("File written.");

// Read all content from the file using File.ReadAllText().
string readContent = File.ReadAllText(filePath, Encoding.UTF8);
Console.WriteLine("--- File Content ---");
Console.WriteLine(readContent);

// Read the file line by line using File.ReadAllLines().
string[] lines = File.ReadAllLines(filePath, Encoding.UTF8);
Console.WriteLine($"Line count: {lines.Length}");
foreach (string line in lines)
{
    Console.WriteLine($"  [{line}]");
}

// Append text to the file using File.AppendAllText().
File.AppendAllText(filePath, "\nLine 4: Appended", Encoding.UTF8);

// Verify the content after appending.
string afterAppend = File.ReadAllText(filePath);
Console.WriteLine($"Line count (after append): {File.ReadAllLines(filePath).Length}");

// Clean up by deleting the file.
File.Delete(filePath);
Console.WriteLine("File deleted.");

File.ReadAllText() and File.WriteAllText() load the entire file into memory at once. For large files (tens of MB or more), this may cause an out-of-memory error. When working with large files, consider using StreamReader / StreamWriter, which process the file one line at a time.

If you omit the encoding parameter, the default in .NET 5 and later is UTF-8 (without BOM). To work with other encodings such as Shift-JIS, specify Encoding.GetEncoding("shift_jis"). For checking file existence, deleting, and copying files, see File.Exists() / Delete() / Copy().