File.Exists() / File.Delete() / File.Copy()

using System.IO;

// Checks whether a file exists (returns true or false).
bool exists = File.Exists(string path);

// Deletes a file. Does nothing if the file does not exist.
File.Delete(string path);

// Copies a file.
File.Copy(string sourceFileName, string destFileName);

// Copies a file (overwrite allowed when true).
File.Copy(string sourceFileName, string destFileName, bool overwrite);

// Moves a file (also used for renaming).
File.Move(string sourceFileName, string destFileName);

// Moves a file (overwrite allowed when true, .NET 5 and later).
File.Move(string sourceFileName, string destFileName, bool overwrite);

Program.cs

using System;
using System.IO;

string source = "original.txt";
string copyDest = "copy.txt";
string moveDest = "moved.txt";

File.WriteAllText(source, "This is a test file.");

Console.WriteLine(File.Exists(source)); // True
Console.WriteLine(File.Exists("none.txt")); // False

// Copies a file.
File.Copy(source, copyDest);
Console.WriteLine($"Copy done: {File.Exists(copyDest)}"); // True

// Overwrite requires overwrite: true.
File.Copy(source, copyDest, overwrite: true);
Console.WriteLine("Overwrite copy done.");

// Moves (or renames) a file.
File.Move(copyDest, moveDest);
Console.WriteLine($"Copy dest after move: {File.Exists(copyDest)}"); // False
Console.WriteLine($"Move dest after move: {File.Exists(moveDest)}"); // True

DateTime lastWrite = File.GetLastWriteTime(source);
Console.WriteLine($"Last write: {lastWrite:yyyy/MM/dd HH:mm:ss}");

// Deletes files.
File.Delete(source);
File.Delete(moveDest);
Console.WriteLine($"After delete: {File.Exists(source)}"); // False

Run the following command:

dotnet script file_exists_delete_copy.csx
True
False
Copy done: True
Overwrite copy done.
Copy dest after move: False
Move dest after move: True
Last write: 2024/01/15 14:30:00
After delete: False

A pattern for creating a timestamped backup before updating a configuration file, then cleaning up old backups.

FileBackup.cs

using System;
using System.IO;

string configFile = "config.json";
string backupFile = $"config_{DateTime.Now:yyyyMMdd_HHmmss}.bak";

File.WriteAllText(configFile, "{\"theme\": \"dark\"}");
Console.WriteLine("Config file created.");

if (File.Exists(configFile))
{
    File.Copy(configFile, backupFile);
    Console.WriteLine($"Backup created: {backupFile}");
}

// Updates the configuration file.
File.WriteAllText(configFile, "{\"theme\": \"light\", \"fontSize\": 14}");
Console.WriteLine("Config file updated.");

// Verifies the backup exists.
Console.WriteLine($"Backup exists: {File.Exists(backupFile)}");
Console.WriteLine($"Backup write time: {File.GetLastWriteTime(backupFile):yyyy/MM/dd HH:mm:ss}");

// Cleanup.
File.Delete(configFile);
File.Delete(backupFile);

Run the following command:

dotnet script file_backup.csx
Config file created.
Backup created: config_20240115_143045.bak
Config file updated.
Backup exists: True
Backup write time: 2024/01/15 14:30:45

A pattern that combines File.Exists() with exception handling to safely handle TOCTOU (time-of-check to time-of-use) race conditions.

SafeFileOps.cs

using System;
using System.IO;

// Safe file delete: existence check + exception handling
static bool TryDeleteFile(string path)
{
    try
    {
        File.Delete(path);
        return true;
    }
    catch (UnauthorizedAccessException ex)
    {
        Console.WriteLine($"Access denied: {ex.Message}");
        return false;
    }
    catch (IOException ex)
    {
        Console.WriteLine($"IO error: {ex.Message}");
        return false;
    }
}

// Safe file copy: checks source before overwriting
static bool TryCopyFile(string src, string dest, bool overwrite = false)
{
    if (!File.Exists(src))
    {
        Console.WriteLine($"Source not found: {src}");
        return false;
    }
    try
    {
        File.Copy(src, dest, overwrite);
        return true;
    }
    catch (IOException ex)
    {
        Console.WriteLine($"Copy error: {ex.Message}");
        return false;
    }
}

// Test
File.WriteAllText("source.txt", "data");
bool ok1 = TryCopyFile("source.txt", "dest.txt");
bool ok2 = TryCopyFile("none.txt", "dest.txt"); // fails
bool ok3 = TryDeleteFile("source.txt");
bool ok4 = TryDeleteFile("dest.txt");
Console.WriteLine($"Copy1: {ok1}, Copy2: {ok2}, Delete1: {ok3}, Delete2: {ok4}");

Run the following command:

dotnet script safe_file_ops.csx
Source not found: none.txt
Copy1: True, Copy2: False, Delete1: True, Delete2: True

Common Mistake 1: IOException When Overwriting Without overwrite Flag

If the destination file already exists, File.Copy() throws an IOException unless overwrite is specified.

using System;
using System.IO;

File.WriteAllText("a.txt", "data1");
File.WriteAllText("b.txt", "data2"); // Destination file already exists.

// NG: Throws IOException because the destination exists.
File.Copy("a.txt", "b.txt");

Run the following command:

dotnet run
Unhandled exception. System.IO.IOException: The file 'b.txt' already exists.

The following example demonstrates this:

using System;
using System.IO;

File.WriteAllText("a.txt", "data1");
File.WriteAllText("b.txt", "data2");

// OK: Pass overwrite: true to allow overwriting.
File.Copy("a.txt", "b.txt", overwrite: true);
Console.WriteLine("Overwrite copy done.");

// Cleanup.
File.Delete("a.txt");
File.Delete("b.txt");

Run the following command:

dotnet run
Overwrite copy done.

Common Mistake 2: Deleting a Read-Only File

Trying to delete a read-only file with File.Delete() throws an UnauthorizedAccessException.

using System;
using System.IO;

File.WriteAllText("readonly.txt", "data");
File.SetAttributes("readonly.txt", FileAttributes.ReadOnly);

// NG: Cannot delete a read-only file.
File.Delete("readonly.txt"); // UnauthorizedAccessException is thrown.

Run the following command:

dotnet run
Unhandled exception. System.UnauthorizedAccessException: Access to the path 'readonly.txt' is denied.

The following example demonstrates this:

using System;
using System.IO;

File.WriteAllText("readonly.txt", "data");
File.SetAttributes("readonly.txt", FileAttributes.ReadOnly);

// OK: Remove the read-only attribute before deleting.
File.SetAttributes("readonly.txt", FileAttributes.Normal);
File.Delete("readonly.txt");
Console.WriteLine("Deleted.");

Run the following command:

dotnet run
Deleted.

File.Exists() has a TOCTOU (time-of-check to time-of-use) race condition issue. Another process may delete the file between the existence check and the actual operation, so always combine existence checks with exception handling.

File.Delete() does nothing when the file does not exist, but throws an UnauthorizedAccessException for read-only files. For reading and writing file contents, see File.ReadAllText() / WriteAllText(). For line-by-line stream processing, see StreamReader / StreamWriter.