C# Input, Conversion, and Validation

Values written in source code are under the programmer's control. Input from a keyboard, file, form, or network request is not. It may be blank, misspelled, too large, or shaped differently from what the program expects. Treat external input as text that must earn its way into the program's trusted state.

Console.ReadLine waits for a line and returns its text without the Enter key. The console does not know that 42 is intended as an age; it returns the two characters as a string. Conversion gives the program a numeric value on which arithmetic is meaningful.

A cast converts between compatible representations already understood by the type system, such as int to double. Parsing interprets the characters in text according to a format. The string "42" is not an int hidden behind a label, so (int)input is not the right operation.

int.Parse returns an integer when the text is valid but throws an exception when it is not. int.TryParse instead returns true or false and places a successful result in an out variable. Invalid user typing is expected, so TryParse usually gives console and form code a clearer control flow.

TryParse answers whether text can represent an int. It does not answer whether the value makes sense for your program. -8 is a valid integer format but not a valid item quantity. After parsing, apply domain rules such as a minimum, maximum, allowed set, or relationship to another value.

The example uses && so every required condition must be true. C# evaluates the parts from left to right and stops as soon as the whole expression must be false. That means age rules are considered only after parsing succeeded.

using System;

class Program
{
  static void Main()
  {
      string quantityText = "12";
      int quantity;

      if (!int.TryParse(quantityText, out quantity))
      {
          Console.WriteLine("Quantity must be a whole number.");
      }
      else if (quantity < 1 || quantity > 50)
      {
          Console.WriteLine("Quantity must be between 1 and 50.");
      }
      else
      {
          Console.WriteLine($"Accepted quantity: {quantity}");
      }
  }
}

Accepted quantity: 12

A useful validation message states the requirement: Enter a whole number from 1 to 50. A vague message such as Invalid input forces the user to guess. Do not echo secrets or internal exception details into user-facing messages.

Interactive applications often repeat the prompt until input is valid or the user cancels. That repetition needs a loop, which comes soon. For now, one attempt is enough to understand the boundary: raw text enters, parsing checks its shape, domain validation checks its meaning, and only then does the program act on it.