How Can I Read a File in Use by Another Program?

Accessing Files Simultaneously

Reading a file actively being written to by another program presents a unique challenge. Standard file reading methods often fail, throwing exceptions indicating the file's locked status.

Solutions for Concurrent File Access

To successfully read such a file, you need a strategy that accommodates concurrent access. A common solution involves opening the file in a mode that permits both reading and writing.

Implementation in C#/.NET

The following C# code snippet demonstrates this approach using FileStream and StreamReader:

<code class="language-csharp">using System;
using System.IO;

namespace ConcurrentFileReader
{
    class Program
    {
        static void Main(string[] args)
        {
            string filePath = "c:\test.txt";

            // Open the file for reading and writing concurrently
            FileStream fileStream = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
            StreamReader reader = new StreamReader(fileStream);

            // Read and process the file line by line
            while (!reader.EndOfStream)
            {
                string line = reader.ReadLine();
                // Process the line
                Console.WriteLine(line);
            }

            // Close resources
            reader.Close();
            fileStream.Close();
        }
    }
}</code>

The key is FileShare.ReadWrite. This ensures the file is opened in a shared mode, allowing simultaneous read and write operations without interruption. The program can now read the file's contents while another process continues writing to it. Note that the data read might be incomplete or reflect only a portion of the file's contents at a given moment, depending on the writing process's activity.

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