C# - File I/O


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 class that are used for performing various operation with files, like creating and deleting files, reading from or writing to a file, closing a file etc.

The following table shows some commonly used non-abstract classes in the System.IO namespace:

I/O ClassDescription
BinaryReaderReads primitive data from a binary stream.
BinaryWriterWrites primitive data in binary format.
BufferedStreamA temporary storage for a stream of bytes.
DirectoryHelps in manipulating a directory structure.
DirectoryInfoUsed for performing operations on directories.
DriveInfoProvides information for the drives.
FileHelps in manipulating files.
FileInfoUsed for performing operations on files.
FileStreamUsed to read from and write to any location in a file.
MemoryStreamUsed for random access to streamed data stored in memory.
PathPerforms operations on path information.
StreamReaderUsed for reading characters from a byte stream.
StreamWriterIs used for writing characters to a stream.
StringReaderIs used for reading from a string buffer.
StringWriterIs used for writing into a string buffer.

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, for creating a FileStream object F for reading a file named sample.txt:

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

The FileMode enumerator defines various methods for opening files. The members of the FileMode enumerator are:

  • Append: It opens an existing file and puts cursor at the end of file, or creates the file, if the file does not exist.

  • Create: It creates a new file.

  • CreateNew: It specifies to the operating system, that it should create a new file.

  • Open: It opens an existing file.

  • OpenOrCreate: It specifies to the operating system that it should open a file if it exists, otherwise it should create a new file.

  • Truncate: It opens an existing file and truncates its size to zero bytes.


FileAccess enumerators have members: Read, ReadWrite and Write.


FileShare enumerators have the following members:

  • Inheritable: It allows a file handle to pass inheritance to the child processes

  • None: It declines sharing of the current file

  • Read: It allows opening the file for reading

  • ReadWrite: It allows opening the file for reading and writing

  • Write: It allows opening the file for writing


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.Position = 0;

            for (int i = 0; i <= 20; i++)
                Console.Write(F.ReadByte() + " ");

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

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.

We will discuss these classes and the operations they perform, in the following sections. Please click the links provided to get to the individual sections:

Topic and Description
Reading from and Writing into Text files
It involves reading from and writing into text files. The StreamReader and StreamWriter class helps to accomplish it.
Reading from and Writing into Binary files
It involves reading from and writing into binary files. The BinaryReader and BinaryWriter class helps to accomplish this.
Manipulating the Windows file system
It gives a C# programamer the ability to browse and locate Windows files and directories.