Putting it together
Combine all parts into a working calculator.
Error handling
In the final lesson we'll add error handling to the calculator. We expect two kinds of errors:
- FormatException: thrown by
double.Parseif the user enters letters instead of a number. - DivideByZeroException: thrown by the method if the divisor is zero.
To catch these errors we put the risky code inside try and an error message inside catch. The program then shows a clear message instead of crashing.
Project complete
Congratulations! You've completed the five-lesson calculator project. Along the way you learned to:
- design a
Calculatorclass withclass - read input with
Console.ReadLineanddouble.Parse - write arithmetic methods and throw
DivideByZeroException - build a menu loop with
switchandwhile - handle errors with
try/catch
These patterns are used the same way in real-world applications.
Exercise Add error handling with try/catch
Wrap the loop body in a try block. Add a default branch to the switch that throws new FormatException("Invalid operation"); for unknown operators. Add two catch blocks: catch (DivideByZeroException) prints Console.WriteLine("Division by zero error"); and catch (FormatException) prints Console.WriteLine("Invalid input");. The program now catches errors without crashing.
Show example solution
Wrap the loop body in try and add two catch blocks.
Example solution:
using System;
class Calculator
{
public double Add(double a, double b) { return a + b; }
public double Subtract(double a, double b) { return a - b; }
public double Multiply(double a, double b) { return a * b; }
public double Divide(double a, double b)
{
if (b == 0) throw new DivideByZeroException("Division by zero");
return a / b;
}
}
class Program
{
static void Main()
{
var calc = new Calculator();
// Test cases: (operator, a, b) — some are invalid
string[][] tests = {
new string[] { "+", "20", "5" },
new string[] { "/", "20", "0" },
new string[] { "?", "20", "5" }
};
int i = 0;
while (i < tests.Length)
{
try
{
string op = tests[i][0];
double a = double.Parse(tests[i][1]);
double b = double.Parse(tests[i][2]);
double result = 0;
switch (op)
{
case "+":
result = calc.Add(a, b);
break;
case "-":
result = calc.Subtract(a, b);
break;
case "*":
result = calc.Multiply(a, b);
break;
case "/":
result = calc.Divide(a, b);
break;
default:
throw new FormatException("Invalid operation");
}
Console.WriteLine(result);
}
catch (DivideByZeroException)
{
Console.WriteLine("Division by zero error");
}
catch (FormatException)
{
Console.WriteLine("Invalid input");
}
i++;
}
}
}Test yourself
-
Which exception is thrown when an invalid number is entered?
- FormatException
- DivideByZeroException
- IndexOutOfRangeException
- InvalidOperationException
Show answer
FormatException — Correct; double.Parse throws FormatException on invalid format.
-
Match each exception or construct to its meaning
- FormatException
- Invalid number format
- DivideByZeroException
- Division by zero
- try
- Protected code block
- catch
- Catching the error
-
Start the catch block
try { result = calc.Divide(a, b); } ____ (DivideByZeroException) { Console.WriteLine("Division by zero error"); }Show answer
try { result = calc.Divide(a, b); } catch (DivideByZeroException) { Console.WriteLine("Division by zero error"); }