Line Filters in C#

Here’s the translation of the Go line filter program to C#, with explanations in Markdown format suitable for Hugo:

A line filter is a common type of program that reads input on stdin, processes it, and then prints some derived result to stdout. grep and sed are common line filters.

Here’s an example line filter in C# that writes a capitalized version of all input text. You can use this pattern to write your own C# line filters.

using System;
using System.IO;

class LineFilter
{
    static void Main()
    {
        // Reading from standard input
        using (StreamReader reader = new StreamReader(Console.OpenStandardInput()))
        {
            string line;
            // Read line by line until the end of the stream
            while ((line = reader.ReadLine()) != null)
            {
                // Convert the line to uppercase
                string upperCaseLine = line.ToUpper();

                // Write out the uppercased line
                Console.WriteLine(upperCaseLine);
            }
        }

        // Check for errors during reading
        if (Console.Error != null)
        {
            Console.Error.WriteLine("An error occurred while reading the input.");
            Environment.Exit(1);
        }
    }
}

In this C# version:

1. We use StreamReader to read from the standard input (Console.OpenStandardInput()).

2. We read the input line by line using a while loop and the ReadLine() method.

3. Each line is converted to uppercase using the ToUpper() method.

4. The uppercased line is then written to the console using Console.WriteLine().

5. Error handling is done by checking if Console.Error is not null after the reading process.

To try out our line filter, first make a file with a few lowercase lines.

$ echo 'hello'   > /tmp/lines
$ echo 'filter' >> /tmp/lines

Then use the line filter to get uppercase lines. Assuming you’ve compiled the C# program into an executable named LineFilter.exe:

$ cat /tmp/lines | ./LineFilter.exe
HELLO
FILTER

This example demonstrates how to create a simple line filter in C# that reads from standard input, processes each line, and writes the result to standard output.