Question

I’d like my app to read from files specified by command-line argument or from standard in, so the user can use it myprogram.exe data.txt or otherprogram.exe | myprogram.exe. How can I do this in C#?

---

In Python, I’d write

import fileinput
for line in fileinput.input():
    process(line)

This iterates over the lines of all files listed in sys.argv[1:], defaulting to sys.stdin if the list is empty. If a filename is ‘-‘, it is also replaced by sys.stdin.

Perl’s <> and Ruby’s ARGF are similarly useful .

Answer 1

stdin is exposed to you as a TextReader through Console.In. Just declare a TextReader variable for your input that either uses Console.In or the file of your choosing and use that for all your input operations.

static TextReader input = Console.In;
static void Main(string[] args)
{
    if (args.Any())
    {
        var path = args[0];
        if (File.Exists(path))
        {
            input = File.OpenText(path);
        }
    }

    // use `input` for all input operations
    for (string line; (line = input.ReadLine()) != null; )
    {
        Console.WriteLine(line);
    }
}

Otherwise if refactoring to use this new variable would be too expensive, you could always redirect Console.In to your file using Console.SetIn().

static void Main(string[] args)
{
    if (args.Any())
    {
        var path = args[0];
        if (File.Exists(path))
        {
            Console.SetIn(File.OpenText(path));
        }
    }

    // Just use the console like normal
    for (string line; (line = Console.ReadLine()) != null; )
    {
        Console.WriteLine(line);
    }
}