Introduction

At execution time programs can run into unspecified behavior, such as
- having to divide by zero,
- having to access an array at an index greater than its length.
For example, the following instructions would compile just fine, but at execution time the program would “explode”:
```
int zero = 0;
Console.WriteLine($"Let's divide by zero: {1 / zero}.");
```
```
int[] test = new int[2];
test[2] = 3;
```
- In the first case, a “System.DivideByZeroException has been thrown” error message would be displayed.
- In the second case, a “System.IndexOutOfRangeException has been thrown” error message would be displayed.
- Those are examples of exceptions thrown by operations.
Methods can also throw exceptions. For example, the following statement:
```
int x = int.Parse("This is not a number.");
```
will display a “System.FormatException has been thrown” error message. This is because the Parse method can throw an exception.
Of course, a programmer would not purposely introduce such strange instructions in their code, but they may arise after interacting with the “outside world”, that is, a user, file, or other external factor.
C# allows exception handling, which are ways of recovering when such exceptions are thrown, so that the program can keep on executing. Stated differently, they instruct the program what to do, for example, if it is asked to perform a division by 0. This is handled by catch blocks.
C# also allows finally blocks, which contain code executed unconditionally, that is, regardless of whether an exception was thrown or not.

Syntax and Rules for `try-catch-finally` Statements

In a first approximation, the syntax of a try-catch-finally statement is as follows:

try{
    <statement block 1>
}
catch{
    <statement block 2>
}
finally{
    <statement block 3>
}

When executed, <statement block 1> will be executed statement by statement. If, at any point, one of the statement in <statement block 1> throws an exception, then the rest of the statements in <statement block 1> will be discarded and <statement block 2> will execute. After all of the statements in <statement block 1> have executed, or after all of the statements in <statement block 2> have executed, then <statement block 3> will execute.
A simple example is
```
try
{
    Console.WriteLine("Enter a number.");
    Console.WriteLine($"Your number is {int.Parse(Console.ReadLine())}.");
}
catch
{
    Console.WriteLine("Something was off.");
}
finally
{
    Console.WriteLine("Did it work?");
}
```
- If the user enters a string that contains only numbers (say, “10”), then the program will display “Your number is 10.” then “Did it work?“.
- If the user enters a string that does not contain only numbers (say, “No”), then the program will display “Something was off.” then “Did it work?“.
Both the catch and the finally parts of the statement are optional: they can be both present, or only one can occur in the try block statement (however, you have to have one or the other).
A try block can have multiple catches, as follows:
```
try
{
    Console.WriteLine("Enter a number");
    int uInput = int.Parse(Console.ReadLine());
    Console.WriteLine($"Your number is {uInput}.");
    Console.WriteLine($"Ten divided by your number is {10 / uInput}.");
 
}
catch (DivideByZeroException)
{
    Console.WriteLine("You tried to divide by zero.");
}
catch (FormatException)
{
    Console.WriteLine("You have tried to convert a string "
      + " containing non-numerical characters to a number.");
}
finally
{
    Console.WriteLine("Did it work?");
}
```
- This allows a more fine-grained handling of the exceptions that can be thrown.
- In the example, if a DivideByZeroException exception is thrown, it is because the user entered “0” and the operation {10 / uInput} failed. In this case, we can display an appropriate error message (“You tried to divide by zero”).
- In the example, if a FormatException exception is thrown, it is because the user entered a string containing non-numerical characters, and we can similarly return an appropriate error message.
- Writing catch{…} is the same as catch (Exception){…}: by default, a catch block catches all the exceptions that can be thrown, not the exceptions of a particular class. Note that, if specifying multiple catch blocks, the order matters, as a catch (Exception), if placed first, will always execute before the catch blocks put after.

Exception Class and Objects

Technically speaking, an exception is an object in a particular class that inherits from the exception class.
We can assign an identifier to it in the catch block, to be able to access some of its properties, such as the Message and a StackTrace properties.
For example, the IndexOutOfRangeException object returned when trying to access an array outside of its bound can be named ex and used to display particular information:
```
try
{
    int[] test = new int[10];
    for (int i = 0; i <= test.Length; i++)
    {
        test[i] = i;
    }
}
catch (IndexOutOfRangeException ex)
{
    Console.WriteLine(ex.Message);
    Console.WriteLine(ex.StackTrace);
}
```
- When the statement test[10] = 10; gets executed, the exception is thrown, named ex, and we display its message (“Index was outside the bounds of the array.”) and StackTrace (“at Program.Main (System.String[] args) [0x0000f] in <path>/Program.cs:<line>”, with <path> the path of the Program.cs file, and <line> the line where the error occurs).

Purpose of the `finally` Block

The difference between
```
try{
    <statement block 1>
}
catch{
    <statement block 2>
}
finally{
    <statement block 3>
}
```
and
```
try{
    <statement block 1>
}
catch{
    <statement block 2>
}
<statement block 3>
```
is that in the second case, <statement block 3> may be skipped if <statement block 1> or <statement block 2> return a value, throw an exception that is not caught, or break the flow of control (using for example break;). In the first case, <statement block 3> will always¹ get executed, no matter which block gets executed and even if it breaks the control flow or throws another exception.

For example,

using System;
 
class Game
{
  public static bool Guess(string guessP)
  {
    const int VALUE_TO_GUESS = 12;
    try
    {
      int guessV = int.Parse(guessP);
      if (guessV == VALUE_TO_GUESS)
      {
        Console.WriteLine("You guessed it!");
        return true;
      }
      else
      {
        Console.WriteLine("Try again!");
        return false;
      }
    }
    catch (FormatException)
    {
      Console.WriteLine(
        "Please, provide a string containing only numbers"
      );
      return false;
    }
    finally
    {
      Console.WriteLine("Thank you for playing!");
    }
  }
}

(Download this code) will always display “Thank you for playing!“. If this last statement was not in the finally block, but was simply inserted after the try-catch statement, then this message would actually never be displayed.

Scoping in `try-catch-finally` Statements

Understanding the scope of statements in try-catch-finally statements can be tricky.
The general rules are:
- Variables declared in try, catch or finally blocks will not be accessible outside of them,
- Variables whose values are set in the try block will keep the values they had on the specific line where the try block threw an exception.
For example, in the following code,
```
int zero = -1;
try
{
    zero = 0;
    int x = 1 / zero;
    zero = 2;
}
catch (DivideByZeroException)
{
    Console.WriteLine("You tried to divide by " + zero + ".");
    zero = 3;
}
finally
{
    Console.WriteLine("The variable holds " + zero + ".");
}
```
- This program will display
```
You tried to divide by 0.
The variable holds 3.
```
- The variable x would not be accessible to the catch or finally blocks.
- If we were to remove the zero = 0; statement, then the program would display “The variable holds 2.”. This is because the statement zero = 3; only executes if the exception is thrown; similarly, the statement zero = 2 only executes if the exception isn’t thrown in the line above it.

When To Use `try-catch` and When To Use `TryParse`?

If something goes wrong in a method, that method can either return some error code or throw an exception.
Returning an error code means possibly cluttering the signature of the method with some extra parameters, as in the TryParse methods.
TryParse is “baking in” a way of signaling that something went wrong because
1. This type of error is simple, common, and predictable,
2. It decided not to care about why the parsing fails (it can be either because the input is null, because it is not in valid format, or because it produces an overflow).

However, exceptions can handle those cases differently thanks to different catch blocks:

Console.WriteLine("Test with" +
    "\n\t- nothing (ctrl + d on linux, ctrl + z on windows), " +
    "\n\t- \"No\"," +
    "\n\t-  " + int.MaxValue + "+ 1 = 2147483648.");
try
{
    int.Parse(Console.ReadLine());
}
catch (ArgumentNullException)
{
    Console.WriteLine("No argument provided.");
}
catch (FormatException)
{
    Console.WriteLine("The string does not contain only number characters.");
}
catch (OverflowException)
{
    Console.WriteLine("The number is greater than what an integer can store.");
}

So, in summary, TryParse is in general better if there is no need to handle the different exceptions differently.

Throwing an Exception

You can explicitly throw an exception by
- Creating an Exception object,
- Throwing it, using the throw keyword.

For example, the property setter in the following class can explicitly throw an ArgumentOutOfRangeException object if we try to create a Circle object with a negative diameter:

using System;
 
class Circle
{
  private decimal diameter;
  public decimal Diameter
  {
    get { return diameter; }
    set
    {
      if (value <= 0)
      {
        throw new ArgumentOutOfRangeException();
      }
      else
      {
        diameter = value;
      }
    }
  }
 
  public Circle(decimal dP)
  {
    Diameter = dP;
  }
 
  public override string ToString()
  {
    return "Diameter: " + diameter;
  }
}

To use this class properly, every time the Diameter value is set (using the set accessor, possibly via the constructor), a try-catch statement should be used to handle a possible exception, with possibly a loop around it, as follows:

using System;
 
class Program
{
  static void Main(string[] args)
  {
    Circle circle1 = new Circle(1);
    Console.WriteLine(circle1);
 
    try
    {
      circle1 = new Circle(-10);
      Console.WriteLine("circle1: " + circle1);
    }
    catch (ArgumentOutOfRangeException)
    {
      Console.WriteLine($"Error: value was out of range.");
    }
    Console.WriteLine(circle1);
 
    bool circle_modified = false;
    do
    {
      try
      {
        Console.WriteLine("Enter the circle new diameter.");
        int uInput = int.Parse(Console.ReadLine());
        circle1.Diameter = uInput;
        Console.WriteLine(circle1);
        circle_modified = true;
      }
      catch (ArgumentOutOfRangeException)
      {
        Console.WriteLine(
          $"Error: value was out of range."
        );
      }
      catch
      {
        Console.WriteLine("Something went wrong.");
        throw;
      }
    } while (!circle_modified);
  }
}

In the last catch block above, the throw; (without argument) will pass the exception to the calling environment. It is indeed possible to catch the exception, do something with it (typically, log it or display an error message), and then “pass” that exception to the surrounding environment.

That is, unless the program crashes or loops forever. ↩

ℙrinℂomℙ

Explorer

Exceptions

Introduction

Syntax and Rules for `try-catch-finally` Statements

Exception Class and Objects

Purpose of the `finally` Block

Scoping in `try-catch-finally` Statements

When To Use `try-catch` and When To Use `TryParse`?

Throwing an Exception

Graph View

Table of Contents

Backlinks

ℙrinℂomℙ

Explorer

Exceptions

Introduction

Syntax and Rules for try-catch-finally Statements

Exception Class and Objects

Purpose of the finally Block

Scoping in try-catch-finally Statements

When To Use try-catch and When To Use TryParse?

Throwing an Exception

Footnotes

Graph View

Table of Contents

Backlinks

Syntax and Rules for `try-catch-finally` Statements

Purpose of the `finally` Block

Scoping in `try-catch-finally` Statements

When To Use `try-catch` and When To Use `TryParse`?