File.AppendAllText(String, String) Method in C# with Examples

File.AppendAllText(String, String) is an inbuilt File class method which is used to append the specified string to the given file if that file exists else creates a new file and then appending is done. It also closes the file.
Syntax:

public static void AppendAllText (string path, string contents);

Parameter: This function accepts two parameters which are illustrated below: 

• path: This is the file where given contents are going to be appended.
• contents: This is the specified contents which is to be appended to the file.

Exceptions:

• ArgumentException: The path is a zero-length string, contains only white space, or one or more invalid characters as defined by InvalidPathChars.
• ArgumentNullException: The path is null.
• PathTooLongException: The given path, file name, or both exceed the system-defined maximum length.
• DirectoryNotFoundException: The given path is invalid i.e, the directory doesn’t exist or it is on an unmapped drive.
• IOException: An I/O error occurred while opening the file.
• UnauthorizedAccessException: The path specified a file that is read-only. OR this operation is not supported on the current platform. OR the path specified a directory. OR the caller does not have the required permission.
• NotSupportedException: The path is in an invalid format.
• SecurityException: The caller does not have the required permission.

Below are the programs to illustrate the File.AppendAllText(String, String) method.
Program 1: Before running the below code, a file is created with some contents shown below:

CSharp

// C# program to illustrate the usage
// of File.AppendAllText() method

// Using System, System.IO,
// and System.Text namespaces
using System;
using System.IO;
using System.Text;

class GFG {
    // Main() method
    public static void Main()
    {
        // Creating a file
        string myfile = @"file.txt";

        // Adding extra texts
        string appendText = "is a CS portal." + Environment.NewLine;
        File.AppendAllText(myfile, appendText);

        // Opening the file to read from.
        string readText = File.ReadAllText(myfile);
        Console.WriteLine(readText);
    }
}

Executing:

mcs -out:main.exe main.cs
mono main.exe
GeeksforGeeks
is a CS portal.

After running the above code, above output is shown and the file contents become like shown below:

Program 2: Initially no file is created but the below code itself creates a new file and appends the specified contents.

CSharp

// C# program to illustrate the usage
// of File.AppendAllText() method

// Using System, System.IO,
// and System.Text namespaces
using System;
using System.IO;
using System.Text;

class GFG {
    // Main() method
    public static void Main()
    {
        // Creating a file
        string myfile = @"file.txt";

        // Checking the existence of file
        if (!File.Exists(myfile)) {
            // Creating a file with below content
            string createText = "GFG" + Environment.NewLine;
            File.WriteAllText(myfile, createText);
        }

        // Adding extra contents
        string appendText = "is a CS portal." + Environment.NewLine;
        File.AppendAllText(myfile, appendText);

        // Opening the file to read from.
        string readText = File.ReadAllText(myfile);
        Console.WriteLine(readText);
    }
}

Executing:

mcs -out:main.exe main.cs
mono main.exe
GFG
is a CS portal.

After running the above code, above output has been shown and a new file created which is shown below: