Exception Handling in C#

C#
LECTURE#12
Abid Kohistani
GC Madyan Swat

Exception Handling
An exception is a problem that arises during the execution of a program.
A C# exception is a response to an exceptional circumstance that arises while a program is running, such as an attempt to
divide by zero.
Exceptions provide a way to transfer control from one part of a program to another. C# exception handling is built
upon four keywords: try, catch, finally, and throw.
try − A try block identifies a block of code for which particular exceptions is activated. It is followed by one or more catch
blocks.
catch − A program catches an exception with an exception handler at the place in a program where you want to handle the
problem. The catch keyword indicates the catching of an exception.
finally − The finally block is used to execute a given set of statements, whether an exception is thrown or not thrown. For
example, if you open a file, it must be closed whether an exception is raised or not.
throw − A program throws an exception when a problem shows up. This is done using a throw keyword.

Syntax
Assuming a block raises an exception, a method catches an exception using a combination of the try and catch keywords.
A try/catch block is placed around the code that might generate an exception.
Code within a try/catch block is referred to as protected code, and the syntax for using try/catch looks like the following:
You can list down multiple catch statements to catch different type of exceptions in case your try block raises more than one
exception in different situations.
try {
// statements causing exception
} catch( ExceptionName e1 ) {
// error handling code
} catch( ExceptionName e2 ) {
} catch( ExceptionName eN ) {
} finally {
// statements to be executed }

Exception Classes in C#
C# exceptions are represented by classes.
The exception classes in C# are mainly directly or indirectly derived from the System.Exception class.
Some of the exception classes derived from the System.
Exception class are the System.ApplicationException and System.SystemException classes.
The System.ApplicationException class supports exceptions generated by application programs. Hence the exceptions defined by
the programmers should derive from this class.
The System.SystemException class is the base class for all predefined system exception.

Sytem.SystemException class
Sr.No. Exception Class & Description
1 System.IO.IOException
Handles I/O errors.
2 System.IndexOutOfRangeException
Handles errors generated when a method refers to an array index out of range.
3 System.ArrayTypeMismatchException
Handles errors generated when type is mismatched with the array type.
4 System.NullReferenceException
Handles errors generated from referencing a null object.
5 System.DivideByZeroException
Handles errors generated from dividing a dividend with zero.
6 System.InvalidCastException
Handles errors generated during typecasting.
7 System.OutOfMemoryException
Handles errors generated from insufficient free memory.
8 System.StackOverflowException
Handles errors generated from stack overflow.

Handling Exceptions
C# provides a structured solution to the exception handling in the form of try and catch blocks. Using these
blocks the core program statements are separated from the error-handling statements.
These error handling blocks are implemented using the try, catch, and finally keywords. Following is an
example of throwing an exception when dividing by zero condition occurs
Example
using System;
namespace ErrorHandlingApplication {
class DivNumbers {
int result;
DivNumbers() { result = 0;
}
public void division(int num1, int num2)
{
try { result = num1 / num2; }
catch (DivideByZeroException e) {
Console.WriteLine("Exception caught: {0}", e);
} finally {
Console.WriteLine("Result: {0}", result);
} }
static void Main(string[] args) {
DivNumbers d = new DivNumbers();
d.division(25, 0);
Console.ReadKey();
} } }
Exception caught:
System.DivideByZeroException: Attempted
to divide by zero.
at ...
Result: 0
output

Creating User-Defined Exceptions
You can also define your own exception. User-defined exception classes are derived from the Exception class. The following
example demonstrates this −
using System;
namespace UserDefinedException {
class TestTemperature {
static void Main(string[] args) {
Temperature temp = new Temperature();
try {
temp.showTemp();
} catch(TempIsZeroException e) {
Console.WriteLine("TempIsZeroException: {0}", e.Message);
} Console.ReadKey();
}
} }
public class TempIsZeroException: Exception {
public TempIsZeroException(string message): base(message) {
}
}
public class Temperature {
int temperature = 0;
public void showTemp() {
if(temperature == 0) {
throw (new TempIsZeroException("Zero Temperature found"));
}
else {
Console.WriteLine("Temperature: {0}", temperature);
}
}
}
TempIsZeroException: Zero Temperature
found
This output will be
displayed

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)
; }
Example
System.ArithmeticException: 'Access denied - You must be at least 18 years old.'Error if age<18
checkAge(20); Access granted - You are old enough!

finally keyword
In programming, sometimes an exception may cause an error which ends the current method.
However, that method might have opened a file or a network that needs to be closed.
So, to overcome such types of problem, C# provides a special keyword named as finally keyword. It is a reserved
keyword in C#.
The finally block will execute when the try/catch block leaves the execution, no matter what condition cause it. It always
executes whether the try block terminates normally or terminates due to an exception.
The main purpose of finally block is to release the system resources.
The finally block follows try/catch block.
Syntax
try
{
// code...
}
// this is optional
Catch
{
// code..
}
finally
{
// code..
}

Important Points:
In C#, multiple finally blocks in the same program are not allowed.
The finally block does not contain any return, continue, break statements because it does not
allow controls to leave the finally block.
You can also use finally block only with a try block means without a catch block but in this
situation, no exceptions are handled.
The finally block will be executed after the try and catch blocks, but before control transfers back
to its origin.

Example
1.using System;
2.public class ExExample
3.{
4. public static void Main(string[] args)
5. {
6. try
7. {
8. int a = 10;
9. int b = 0;
10. int x = a / b;
11. }
12. catch (Exception e) { Console.WriteLine(e); }
13. finally { Console.WriteLine("Finally block is executed"); }
14. Console.WriteLine("Rest of the code");
15. }
16.}
System.DivideByZeroException: Attempted to divide by
zero.
Finally block is executed
Rest of the code
output

Exception Handling in C#

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