File.AppendText() Method in C# with Examples

File.AppendText() is an inbuilt File class method which is used to create a StreamWriter that appends UTF-8 encoded text to an existing file else it creates a new file if the specified file does not exist.
Syntax:

public static System.IO.StreamWriter AppendText (string path);

Parameter: This function accepts a parameter which is illustrated below: 

• path: This is the file where UTF-8 encoded texts are going to be appended. The file is created if it doesn’t already exist.

Exceptions

• UnauthorizedAccessException: The caller does not have the required permission.
• ArgumentException: The path is a zero-length string, contains only white space, or contains 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.
• NotSupportedException: The path is in an invalid format.

Return Value: Returns a stream writer that appends specified UTF-8 encoded texts to the specified file or to a new file.
Below are the programs to illustrate the File.AppendText() method.
Program 1: Before running the below code, a file file.txt is created with some contents which is shown below:

CSharp

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

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

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

        // Appending the given texts
        using(StreamWriter sw = File.AppendText(myfile))
        {
            sw.WriteLine("Gfg");
            sw.WriteLine("GFG");
            sw.WriteLine("GeeksforGeeks");
        }

        // Opening the file for reading
        using(StreamReader sr = File.OpenText(myfile))
        {
            string s = "";
            while ((s = sr.ReadLine()) != null) {
                Console.WriteLine(s);
            }
        }
    }
}

Executing:

mcs -out:main.exe main.cs
mono main.exe
Geeks
Gfg
GFG
GeeksforGeeks

After running the above code, above output is shown and the existing file file.txt becomes like below: 

Program 2: Initially, no file is created and hence below code itself create a file named as file.txt

CSharp

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

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

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

        // Checking the above file
        if (!File.Exists(myfile)) {
            // Creating the same file if it doesn't exist
            using(StreamWriter sw = File.CreateText(myfile))
            {
                sw.WriteLine("GeeksforGeeks");
                sw.WriteLine("is");
                sw.WriteLine("a");
            }
        }

        // Appending the given texts
        using(StreamWriter sw = File.AppendText(myfile))
        {
            sw.WriteLine("computer");
            sw.WriteLine("science");
            sw.WriteLine("portal.");
        }

        // Opening the file for reading
        using(StreamReader sr = File.OpenText(myfile))
        {
            string s = "";
            while ((s = sr.ReadLine()) != null) {
                Console.WriteLine(s);
            }
        }
    }
}

Executing:

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

After running the above code, a new file file.txt is created which is shown below: