Read input safely: text becomes a number only after a check

What should happen when someone types twenty instead of 20? We need to check the text before using it as a duration.

Predict the conversion before using the keyboard

Keyboard input arrives as text. The text "25" and the int value 25 are different kinds of value. The TryParse operation tries to turn suitable text into an int. Start with fixed text so you can see the two results it provides:

using System;

string text = "25";
bool parsed = int.TryParse(text, out int minutes);

Console.WriteLine($"Parsed: {parsed}");
Console.WriteLine($"Minutes: {minutes}");

Read out int minutes as a small recipe for now: create an int variable named minutes, and let TryParse write its result there. The returned bool says whether conversion succeeded. Use the number only on the success path. A later lesson on methods explains parameter passing more deeply.

See the input journey

ReadLine may return null, which takes the missing-input path. Available text is passed to int.TryParse. Failed parsing is reported without using the numeric result. Successful parsing is followed by the nonnegative rule: -5 is rejected, while 25 is accepted. A blank line is text and fails parsing.

Parsing and validation answer different questions. Parsing asks whether the text represents a value of the required type. Our application rule then asks whether that value is allowed. For example, -5 is a valid int, but a negative study duration is not allowed by this planner.

Build a one-question planner

Write your attempt before reading the complete program. Print Study minutes?, read one line, and handle three kinds of result: no line, an invalid whole number, or a parsed number. A parsed number needs its own negative check.

using System;

Console.WriteLine("Study minutes?");
string? text = Console.ReadLine();

if (text == null)
{
    Console.WriteLine("No input received.");
}
else if (int.TryParse(text, out int minutes))
{
    if (minutes < 0)
    {
        Console.WriteLine("Minutes cannot be negative.");
    }
    else
    {
        Console.WriteLine($"Planned {minutes} minutes.");
    }
}
else
{
    Console.WriteLine("Enter a valid whole number.");
}

Console.ReadLine reads a line without its ending newline. Its result can be null when no more input is available. The declaration string? text acknowledges that possibility. Pressing Enter on an otherwise blank line gives empty text; that is different from no line at all. We check null before the remaining conditions to make the distinction visible.

Run the program. When it asks, type 25 and press Enter. The application output is shown below; what you type is input, not another WriteLine instruction:

Study minutes?
Planned 25 minutes.

Practise the unhappy paths

Run again for each row. Each run reads just one line; loops for repeated attempts come next.

Independent change: allow only durations from 0 to 180. Inside the successful parse branch, reject minutes above 180 with a message. Test -1, 0, 180 and 181. Your acceptance condition must include both endpoints. Of these four test inputs, accept 0 and 180. Add this rule after you can explain the existing paths.

Explain back: Why is successful parsing not enough to accept a duration? What two pieces of information does TryParse give us?

References: Microsoft — Console.ReadLine, Int32.TryParse.