C# Input and Output

Console.Write("What's your name? ");
string? name = Console.ReadLine();
Console.WriteLine($"Nice to meet you, {name}!");

• Console.Write keeps the cursor on the same line, which suits prompts.
• ReadLine waits for the user to press Enter and returns everything they typed, without the new line.
• It returns string? (a string that might be null), because input can end, for example when it's redirected from a file. Console.ReadLine() ?? "" replaces null with an empty string.

Everything typed arrives as text, so numbers must be converted:

Console.Write("How many tickets? ");
string? text = Console.ReadLine();

if (int.TryParse(text, out int tickets) && tickets > 0)
{
    Console.WriteLine($"Booking {tickets} tickets");
}
else
{
    Console.WriteLine("Please enter a positive whole number.");
}

TryParse returns false for empty, null or invalid text instead of throwing an exception, so it's the right choice for anything a person types. Use double.TryParse or decimal.TryParse for decimals.

Wrap the prompt in a loop that only ends when the input is acceptable:

int age;
while (true)
{
    Console.Write("Age: ");
    if (int.TryParse(Console.ReadLine(), out age) && age is >= 0 and <= 120)
        break;
    Console.WriteLine("Please enter an age between 0 and 120.");
}

age is >= 0 and <= 120 is a pattern that checks a range in one readable expression. Putting this logic in a helper method, such as ReadInt(string prompt, int min, int max), keeps the rest of the program tidy.

Values typed after the program name arrive in args: running dotnet run -- report.csv 10 gives args[0] == "report.csv" and args[1] == "10". Check args.Length before using them.

• Console.ReadKey() reads a single key press without Enter, useful for menus.
• Console.ForegroundColor = ConsoleColor.Green; changes the text colour, and Console.ResetColor() restores it.
• Console.Clear() clears the screen.

ReadLine returns null when there's no more input. That happens when someone presses Ctrl+Z then Enter on Windows or Ctrl+D on macOS and Linux, and when input comes from a file. You can use it to process any number of lines:

int total = 0;
string? line;
while ((line = Console.ReadLine()) != null)
{
    if (int.TryParse(line, out int n))
        total += n;
}
Console.WriteLine($"Total: {total}");

Run it with dotnet run < numbers.txt and the program reads the file as if someone had typed each line. Redirecting input like this is a quick way to test a console program without typing the same values over and over.

Programs have two output streams. Console.WriteLine writes to standard output, and Console.Error.WriteLine writes to standard error. Both appear in the terminal, but when a user saves your output to a file with > results.txt, error messages still show on screen instead of mixing with the results.

A program also finishes with an exit code: 0 means success and anything else means something went wrong. Scripts and build tools check this number. With top-level statements, you can simply return it:

if (args.Length == 0)
{
    Console.Error.WriteLine("Usage: report <file>");
    return 1;
}
Console.WriteLine($"Reading {args[0]}");
return 0;

In a classic program, declare static int Main(string[] args) and return the code in the same way.