Exceptions

Exceptions are unexpected conditions or errors in our program (e.g. data is not valid, arithmetic error, parsing error, etc.)

For example:

int i = 5;
int j = 0;
Console.WriteLine(i / j);

results in:

Unhandled Exception: System.DivideByZeroException: Attempted to divide by zero.

👎 It also contains awful variable names, but let’s fix one problem at a time.

We can avoid this error with logic:

int i = 5;
int j = 0;

if (j != 0) {
    Console.WriteLine(i / j);
} // end if
else {
   Console.WriteLine("Cannot divide by zero.");
} // end else

Or, we can catch the exception!

int i = 5;
int j = 0;

try { // we'll try to run the code in this block. 
    // if any of it fails, we jump to the catch block.
    Console.WriteLine(i / j);
} // end try
catch { // this code will only run if there was an exception in the try block.
    Console.WriteLine("Cannot divide by zero.");
} // end catch

Accessing Exception data

We can also view the specific exception message:

int i = 5;
int j = 0;

try {
    Console.WriteLine(i / j);
} // end try
catch (Exception ex) {
    Console.WriteLine("Cannot divide by zero.");
    Console.WriteLine(ex.Message);
} // end catch

Which returns:

Cannot divide by zero.              // our message
Attempted to divide by zero.        // the Message in the Exception object

Common Use Cases

We can catch exceptions of many kinds. A common exception is when we parse our user’s input.

For example:

int userAge;
Console.WriteLine("How old are you?");
userAge = int.Parse(Console.ReadLine());

This works perfectly if the user enters a number. But what if the user enters something else, like the word “eight”?

🚨 An exception!

We can move this into a try/catch block:

int userAge;
Console.WriteLine("How old are you?");

try
{
    userAge = int.Parse(Console.ReadLine());
} // end try
catch (Exception)
{
    Console.WriteLine("Invalid input!");
} // end catch

Now, the program doesn’t crash, but instead returns a nice message back to the Console.

We can continue to build on this example by moving the code inside of a loop, which we remain in until the user enters a valid number:

int userAge;
bool validInput = false; // default value, will be set to true if parsing is successful
Console.WriteLine("How old are you?");

while (!validInput)
{
    try
    {
        userAge = int.Parse(Console.ReadLine());
        validInput = true;
    } // end try
    catch (Exception)
    {
        Console.WriteLine("Invalid input! Please re-enter.");
    } // end catch

} // end while

Console.WriteLine("We got to the end!");

An alternative way to deal with parsing exceptions would be to use the .TryParse() method. This returns a bool that is true if the parsing was successful, and false otherwise, and when we call this method, we pass in a string that we want to parse, and an out reference of the variable where the parsed data should be stored.

For example:

string rawInputString = Console.ReadLine();
int userInt;
bool parsingSuccessful = int.TryParse(rawInputString, out userInt);

Throwing our own exceptions

We may also write methods which throw exceptions in certain conditions.

We could have a method containing a segment of code that looks something like:

bool isValid;

// some code executes that sets isValid to true or false

if (isValid) {
    Console.WriteLine("Input was valid.");
} // end if
else {
    throw new Exception("Input is invalid.");
} // end else

The code calling this method would then need to handle the exception this method has thrown.

⚾ Everything that is thrown must be caught!