C# - File I/O

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A file is a collection of data stored in a disk with a specific name and a directory path. When a file is opened for reading or writing, it becomes a stream.

The stream is basically the sequence of bytes passing through the communication path. There are two main streams: the input stream and the output stream. The input stream is used for reading data from file (read operation) and the output stream is used for writing into the file (write operation).

C# I/O Classes

The System.IO namespace has various classes that are used for performing numerous operations with files, such as creating and deleting files, reading from or writing to a file, closing a file etc.

The following list shows some of the commonly used non-abstract classes in the System.IO namespace −

The FileStream Class

The FileStream class in the System.IO namespace helps in reading from, writing to and closing files. This class derives from the abstract class Stream.

You need to create a FileStream object to create a new file or open an existing file. The syntax for creating a FileStream object is as follows −

FileStream <object_name> = new FileStream( <file_name>, <FileMode Enumerator>,
   <FileAccess Enumerator>, <FileShare Enumerator>);

For example, we create a FileStream object F for reading a file named sample.txt as shown −

FileStream F = new FileStream("sample.txt", FileMode.Open, FileAccess.Read,
   FileShare.Read);

Example

The following program demonstrates use of the FileStream class −

using System;
using System.IO;
namespace FileIOApplication {
   class Program {
      static void Main(string[] args) {
         FileStream F = new FileStream("test.dat", FileMode.OpenOrCreate, 
            FileAccess.ReadWrite);
         
         for (int i = 1; i <= 20; i++) {
            F.WriteByte((byte)i);
         }
         F.Position = 0;
         for (int i = 0; i <= 20; i++) {
            Console.Write(F.ReadByte() + " ");
         }
         F.Close();
         Console.ReadKey();
      }
   }
}

Output

When the above code is compiled and executed, it produces the following result −

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 -1

Opening and Closing a File

You can open and read a file using the StreamReader class. This class provides methods like ReadLine() to read data line by line, and it closes the file automatically if you are using the "using" statement to open the file and read the data.

Example

The following example demonstrates how to open a file using StreamReader and read its contents line by line:

using System;
using System.IO;

class Program {
    static void Main() {
        using (StreamReader reader = new StreamReader("data.txt")) {
            string line;
            while ((line = reader.ReadLine()) != null) {
                Console.WriteLine(line);
            }
        }
    }
}

When the above code is compiled and executed, it produces the following result −

[Contents of data.txt displayed line by line]

Create a File

You can create a file by using the File.CreateText() method. This method returns a StreamWriter object that can be used to write text to the file.

Example

The following example demonstrates how to create a file and write some content into it:

using System;
using System.IO;

class Program {
    static void Main() {
        using (StreamWriter writer = File.CreateText("data.txt")) {
            writer.WriteLine("This is the first line.");
            writer.WriteLine("This is the second line.");
        }
    }
}

When the above code is compiled and executed, it produces the following result −

[data.txt file is created with two lines of text]

Append to a File

You can append content to an existing file using the File.AppendText() method. This method returns a StreamWriter object to write text without overwriting existing content.

Example

The following example demonstrates how to append text to a file:

using System;
using System.IO;

class Program {
    static void Main() {
        using (StreamWriter writer = File.AppendText("data.txt")) {
            writer.WriteLine("This line is appended.");
        }
    }
}

When the above code is compiled and executed, it produces the following result −

[data.txt now contains an extra line at the end]

Delete a File

You can delete a file using the File.Delete() method by passing the file path as a parameter.

Example

The following example demonstrates how to delete a file:

using System;
using System.IO;

class Program {
    static void Main() {
        string path = "data.txt";
        if (File.Exists(path)) {
            File.Delete(path);
            Console.WriteLine("File deleted.");
        } else {
            Console.WriteLine("File does not exist.");
        }
    }
}

When the above code is compiled and executed, it produces the following result −

File deleted.

Check if a File Exists

You can check if a file exists using the File.Exists() method, which returns true if the file is found.

Example

The following example demonstrates how to check for file existence:

using System;
using System.IO;

class Program {
    static void Main() {
        string path = "data.txt";
        if (File.Exists(path)) {
            Console.WriteLine("File exists.");
        } else {
            Console.WriteLine("File does not exist.");
        }
    }
}

When the above code is compiled and executed, it produces the following result −

File exists.

Advanced File Operations in C#

The preceding example provides simple file operations in C#. However, to utilize the immense powers of C# System.IO classes, you need to know the commonly used properties and methods of these classes.