C# 03: Files Reading and Writing

How to read and write files with C#.

Namespace

Using the System.IO namespace obtains several classes for reading and writing files:

• File system: File creation, copying, deletion, moving, opening of a file.
• Read: StreamReader a TextReader that reads from a string.
• Write: StreamWriter a TextWriter that writes to a string.

Location

The default location of a file is the currrent directory: .\bin\Debug\ To open a file at a different location, prefix a path string with the @ symbol. @”C:\Users\me\Documents\test.txt”`. (The verbatim string literal, which is prefixed with the @ symbol, handles escape characters without having to escape the escape character).

File

• .OpenText opens a file.
• .CreateText writes a new file, or overwrites an existing file.
• .AppendText preserves an existing file while adding to its contents.

Reading

The StreamReader class provides two methods to read data from file:

• line: .ReadLine returns a line of data as a string.
• string: .Read returns characters from data as a string.

try
    {
    // Initialise a variable to hold a line from the file
    string fileData; 
    
    // Instantiate a Stream Reader Object
    StreamReader fileReader;

    // Open the file 
    fileReader = File.OpenText("test.txt"); 

    // Keep reading lines until reaching the end of the text
    while (!fileReader.EndOfStream)
    {
        fileData = fileReader.ReadLine(); 

        // Do something 
    }

    // Close the connection after reaching the end of the text
    fileReader.Close();
}
catch (Exception exErr)
{
    // Display an error message
    MessageBox.Show(exErr.Message);
}

Writing

Numeric data gets converted to a string.

// Directory locations
string filePath = @"target_file.txt";

// Initialise a new StreamWriter object to write to the file 
// Clean up memory with `using()` syntax
using (StreamWriter fileWriter = new StreamWriter(filePath))
{
    // Write a line to the file
    fileWriter.WriteLine("Hello World!");
}

Append

try
{
    // Directory locations
    string filePath = @"target_file.txt";

    // Initialise a new StreamWriter object to append text to file
    StreamWriter fileAppender;
    fileAppender = File.AppendText(filePath);

    // Write a line to the file
    fileAppender.WriteLine("Append this text");
    fileAppender.Close()
}
catch (Exception exErr)
{
    // Display the error message
    MessageBox.Show(exErr.Message);
}

QED

© Adam Heinz

6 November 2024