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Console input and output ​

Console input and output ​

The Console class (https://learn.microsoft.com/dotnet/api/system.console) provides console operations:

  • Console.Write(value) — writes a value without moving to a new line;
  • Console.WriteLine(value) — writes a value and moves to a new line;
  • Console.ReadLine() — reads a line entered by the user and returns it as string? (null when input has ended);
  • Console.ReadKey() — waits for a single key press;
  • Console.Clear() — clears the console window.

Converting input ​

ReadLine always returns a string, so you must convert it to obtain a number. int.Parse throws a FormatException if the string is not a number, whereas int.TryParse returns a Boolean success value and writes the result to an output parameter:

cs
Console.OutputEncoding = System.Text.Encoding.UTF8;

Console.Write("Enter your birth year: ");
string? input = Console.ReadLine();

if (int.TryParse(input, out int year))
{
    int age = DateTime.Now.Year - year;
    Console.WriteLine($"You turn {age} this year.");
}
else
{
    Console.WriteLine("Invalid value.");
}

Similarly, use double.Parse and double.TryParse for real numbers, and the decimal type with decimal.Parse and decimal.TryParse for monetary amounts. The decimal separator depends on the system's regional settings: a comma (12,5) for Ukrainian and a period (12.5) for English. A program expecting 12,5 will therefore not accept that value on a computer with English settings.

Formatted output ​

Interpolated strings prefixed with $ are convenient for formatted output: expressions in braces are evaluated and inserted into the string, and a format can be specified after a colon (Table 1.5):

cs
double price = 1234.5;
// Prints: Price: 1234,50 UAH
Console.WriteLine($"Price: {price:F2} UAH");

Table 1.5. Numeric format specifiers

FormatExampleResult for 1234,567
F2{x:F2}1234,57 — fixed number of decimal places
N2{x:N2}1 234,57 — with digit grouping separators
E2{x:E2}1,23E+003 — scientific notation
C{x:C}1 234,57 ₴ — currency format
P1{0.25:P1}25,0% — percentage (without a space before %)
,10{x,10:F1}right-aligned in a field 10 characters wide
,-10{s,-10}left-aligned in a field 10 characters wide

All standard numeric format specifiers are documented at https://learn.microsoft.com/dotnet/standard/base-types/standard-numeric-format-strings. Dates and times are formatted similarly: {date:dd.MM.yyyy} displays the date, and {date:HH:mm} the time (https://learn.microsoft.com/dotnet/standard/base-types/custom-date-and-time-format-strings).

Strings can contain escape sequences: \n for a new line, \t for a tab, \" for a quotation mark, and \\ for a backslash. In a string prefixed with @ (@"C:\Temp"), the backslash has no special meaning, which is convenient for file paths.

UTF-8 encoding ​

To display and enter Ukrainian letters correctly in the Windows console, set UTF-8 encoding at the beginning of the program:

cs
Console.OutputEncoding = System.Text.Encoding.UTF8;
Console.InputEncoding = System.Text.Encoding.UTF8;

The Console.ForegroundColor property changes the text color, for example Console.ForegroundColor = ConsoleColor.Red;, and Console.ResetColor() restores the default colors. Do not convey important information through color alone: colors look different in different terminal themes.

Program examples ​

Temperature converter ​

The program asks for a temperature in Celsius and converts it to Fahrenheit and Kelvin. If the user enters something other than a number, the prompt repeats.

cs
Console.OutputEncoding = System.Text.Encoding.UTF8;

double celsius = ReadNumber("Temperature in Celsius: ");
double fahrenheit = celsius * 9 / 5 + 32;
double kelvin = celsius + 273.15;

Console.WriteLine($"In Fahrenheit: {fahrenheit:F1} °F");
Console.WriteLine($"In Kelvin: {kelvin:F2} K");

// Repeats the prompt until the user enters a number.
static double ReadNumber(string prompt)
{
    while (true)
    {
        Console.Write(prompt);
        string? input = Console.ReadLine();
        if (double.TryParse(input, out double value))
        {
            return value;
        }
        Console.WriteLine("That is not a number. Try again.");
    }
}

Repeated input is extracted into the local function ReadNumber; the while (true) loop ends with return as soon as a number is entered. Example session (the user enters abc and -12,5):

Temperature in Celsius: abc
That is not a number. Try again.
Temperature in Celsius: -12,5
In Fahrenheit: 9,5 °F
In Kelvin: 260,65 K

Purchase receipt ​

The program reads a product name, price, and quantity, then prints a receipt as an aligned table. Monetary amounts use decimal, which stores decimal fractions without rounding errors; the m suffix indicates a decimal literal.

cs
Console.OutputEncoding = System.Text.Encoding.UTF8;
Console.InputEncoding = System.Text.Encoding.UTF8;

Console.Write("Product name: ");
string name = Console.ReadLine() ?? "";
Console.Write("Unit price, UAH: ");
decimal price = decimal.Parse(Console.ReadLine() ?? "0");
Console.Write("Quantity: ");
int count = int.Parse(Console.ReadLine() ?? "0");

decimal total = price * count;
decimal vat = total * 0.2m / 1.2m; // 20% VAT included in the price
string line = new string('-', 38);

Console.WriteLine();
Console.WriteLine($"{"Product",-20}{"Qty",6}{"Amount",12}");
Console.WriteLine(line);
Console.WriteLine($"{name,-20}{count,6}{total,12:N2}");
Console.WriteLine(line);
Console.WriteLine($"{"Total due:",-26}{total,12:N2}");
Console.WriteLine($"{"including VAT:",-26}{vat,12:N2}");

The ?? operator substitutes an empty string or "0" if ReadLine returns null. A negative field width (-20) aligns text to the left; a positive width (6, 12) aligns it to the right. Output:

Product name: Headphones
Unit price, UAH: 1299,50
Quantity: 2

Product                Qty      Amount
--------------------------------------
Headphones               2    2 599,00
--------------------------------------
Total due:                    2 599,00
including VAT:                  433,17

Travel time ​

The program calculates how many hours and minutes a trip will take and the estimated arrival time.

cs
Console.OutputEncoding = System.Text.Encoding.UTF8;

Console.Write("Distance, km: ");
int distance = int.Parse(Console.ReadLine() ?? "0");
Console.Write("Average speed, km/h: ");
int speed = int.Parse(Console.ReadLine() ?? "0");

int totalMinutes = distance * 60 / speed; // integer division
int hours = totalMinutes / 60;
int minutes = totalMinutes % 60;          // remainder

DateTime arrival = DateTime.Now.AddMinutes(totalMinutes);
Console.WriteLine($"Travel time: {hours} h {minutes} min");
Console.WriteLine($"Estimated arrival at {arrival:HH:mm}");

Integer division discards the fractional part, and % returns the remainder, so 331 minutes is 5 hours and 31 minutes. AddMinutes adds minutes to the current time. Output (the arrival time depends on when the program runs):

Distance, km: 470
Average speed, km/h: 85
Travel time: 5 h 31 min
Estimated arrival at 16:45

Self-check task

What happens if you enter a speed of 0? Change the program to report an invalid speed using int.TryParse and a speed > 0 check.