C# Exceptions - Try..Catch

C# Exceptions

When executing C# code, different errors can occur: coding errors made by the programmer, errors due to wrong input, or other unforeseeable things.

When an error occurs, C# will normally stop and generate an error message. The technical term for this is: C# will throw an exception (throw an error).

C# try and catch

The try statement allows you to define a block of code to be tested for errors while it is being executed.

The catch statement allows you to define a block of code to be executed, if an error occurs in the try block.

The try and catch keywords come in pairs:

try 
{
  //  Block of code to try
}
catch (Exception e)
{
  //  Block of code to handle errors
}

Consider the following example, where we create an array of three integers:

int[] myNumbers = {1, 2, 3};
Console.WriteLine(myNumbers[10]); // error!

If an error occurs, we can use try...catch to catch the error and execute some code to handle it.

In the following example, we use the variable inside the catch block (e) together with the built-in Message property, which outputs a message that describes the exception:

try
{
  int[] myNumbers = {1, 2, 3};
  Console.WriteLine(myNumbers[10]);
}
catch (Exception e)
{
  Console.WriteLine(e.Message);
}

You can also output your own error message:

try
{
  int[] myNumbers = {1, 2, 3};
  Console.WriteLine(myNumbers[10]);
}
catch (Exception e)
{
  Console.WriteLine("Something went wrong.");
}

Finally

The finally statement lets you execute code, after try...catch, regardless of the result:

try
{
  int[] myNumbers = {1, 2, 3};
  Console.WriteLine(myNumbers[10]);
}
catch (Exception e)
{
  Console.WriteLine("Something went wrong.");
}
finally
{
  Console.WriteLine("The 'try catch' is finished.");
}

The throw keyword

The throw statement allows you to create a custom error.

The throw statement is used together with an exception class. There are many exception classes available in C#: ArithmeticException, FileNotFoundException, IndexOutOfRangeException, TimeOutException, etc:

static void checkAge(int age)
{
  if (age < 18)
  {
    throw new ArithmeticException("Access denied - You must be at least 18 years old.");
  }
  else
  {
    Console.WriteLine("Access granted - You are old enough!");
  }
}

static void Main(string[] args)
{
  checkAge(15);
}

If age was 20, you would not get an exception:

checkAge(20);