Exception Handlin g C#.pptx

EXCEPTION HANDLING
Handling Errors during the Program
Execution

What are Exceptions?
Handling Exceptions
The System.Exception Class
Types of Exceptions and their Hierarchy
Raising (Throwing) Exceptions
Best Practices
2

WHAT ARE EXCEPTIONS?
The exceptions in .NET Framework are classic
implementation of the OOP exception model
Deliver powerful mechanism for centralized handling of
errors and unusual events
Substitute procedure-oriented approach,
in which each function returns error code
Simplify code construction and maintenance
Allow the problematic situations to be
processed at multiple levels
3

HANDLING EXCEPTIONS
In C# the exceptions can be handled by the try-
catch-finally construction
catch blocks can be used multiple times to
process different exception types
4
try
{
// Do some work that can raise an exception
}
catch (DivideByZeroException ex)
{
// Handle the caught DivideByZeroException ex
}
Catch (Exeption ex)
{
// Handle the caught Exception ex
}

HANDLING EXCEPTIONS –
EXAMPLE
5
static void Main()
{
string s = Console.ReadLine();
try
{
Int32.Parse(s);
Console.WriteLine(
"You entered valid Int32 number {0}.", s);
}
catch (FormatException)
{
Console.WriteLine("Invalid integer number!");
}
catch (OverflowException)
{
Console.WriteLine(
"The number is too big to fit in Int32!");
}
}

THE SYSTEM.EXCEPTION
CLASS
Exception is a base class for all exceptions
Important properties:
Message – user-oriented message about error
Source – name of an error source (application or object)
InnerException – inner exception (if called from other)
StackTrace – call stack to the point of exception call
TargetSite – method name which raised an exception
HelpLink – URL-address to information about exception
Data – dictionary with additional information with exception
(IDictionary)
6

EXCEPTION PROPERTIES –
EXAMPLE
7
class ExceptionsExample
{
public static void CauseFormatException()
{
string s = "an invalid number";
Int32.Parse(s);
}
static void Main()
{
try
{
CauseFormatException();
}
catch (FormatException fe)
{
Console.Error.WriteLine("Exception: {0}n{1}",
fe.Message, fe.StackTrace);
}
}
}

EXCEPTION PROPERTIES
The Message property gives brief
description of the problem
The StackTrace property is extremely
useful when identifying the reason
caused the exception
8
Exception caught: Input string was not in a correct
format.
at System.Number.ParseInt32(String s, NumberStyles
style, NumberFormatInfo info)
at System.Int32.Parse(String s)
at ExceptionsTest.CauseFormatException() in
c:consoleapplication1exceptionstest.cs:line 8
at ExceptionsTest.Main(String[] args) in
c:consoleapplication1exceptionstest.cs:line 15

EXCEPTION PROPERTIES (2)
File names and line numbers are
accessible only if the compilation was in
Debug mode
When compiled in Release mode, the
information in the property StackTrace
is quite different:
9
Exception caught: Input string was not in a correct
format.
at System.Number.ParseInt32(String s, NumberStyles
style, NumberFormatInfo info)
at ExceptionsTest.Main(String[] args)

EXCEPTION HIERARCHY
Exceptions in .NET Framework are organized in a
hierarchy
10

TYPES OF EXCEPTIONS
.NET exceptions inherit from System.Exception
The system exceptions inherit from
System.SystemException, e.g.
 System.ArgumentException
 System.NullReferenceException
 System.OutOfMemoryException
 System.StackOverflowException
User-defined exceptions should inherit from
System.ApplicationException
11

HANDLING EXCEPTIONS
When catching an exception of a particular class, all
its inheritors (child exceptions) are caught too
Example:
Handles ArithmeticException and its descendants
DivideByZeroException and OverflowException
12
try
{
// Do some works that can cause an exception
}
catch (System.ArithmeticException)
{
// Handle the caught arithmetic exception
}

FIND THE MISTAKE!
13
static void Main()
{
string s = Console.ReadLine();
try
{
Int32.Parse(s);
}
catch (Exception)
{
Console.WriteLine("Can not parse the number!");
}
{
Console.WriteLine("Invalid integer number!");
}
catch (OverflowException)
{
Console.WriteLine(
"The number is too big to fit in Int32!");
}
}
This should be
last
Unreachable
code
Unreachable
code

HANDLING ALL EXCEPTIONS
All exceptions thrown by .NET managed code
inherit the System.Exception exception
Unmanaged code can throw other exceptions
For handling all exceptions (even unmanaged)
use the construction:
14
try
{
// Do some works that can raise any exception
}
catch
{
// Handle the caught exception
}

THROWING EXCEPTIONS
Exceptions are thrown (raised) by throw keyword in C#
 Used to notify the calling code in case of error or unusual
situation
When an exception is thrown:
 The program execution stops
 The exception travels over the stack until a suitable catch block
is reached to handle it
Unhandled exceptions display
error message
15

HOW EXCEPTIONS WORK?
16
Main()
Method 1
Method 2
Method N
2. Method call
3. Method call
4. Method call
…
Main()
Method 1
Method 2
Method N
8. Find handler
7. Find handler
6. Find handler
…
5. Throw an exception
.NET
CLR

USING THROW KEYWORD
Throwing an exception with an error message:
Exceptions can accept message and cause:
Note: if the original exception is not passed the initial cause of
the exception is lost
17
throw new ArgumentException("Invalid amount!");
try
{
Int32.Parse(str);
}
{
throw new ArgumentException("Invalid number", fe);
}

RE-THROWING EXCEPTIONS
Caught exceptions can be re-thrown again:
18
try
{
Int32.Parse(str);
}
{
Console.WriteLine("Parse failed!");
throw fe; // Re-throw the caught exception
}
{
throw; // Re-throws the last caught exception
}

THROWING EXCEPTIONS –
EXAMPLE
19
public static double Sqrt(double value)
{
if (value < 0)
throw new System.ArgumentOutOfRangeException(
"Sqrt for negative numbers is undefined!");
return Math.Sqrt(value);
}
static void Main()
{
try
{
Sqrt(-1);
}
catch (ArgumentOutOfRangeException ex)
{
Console.Error.WriteLine("Error: " + ex.Message);
throw;
}
}

CHOOSING THE EXCEPTION
TYPE
When an invalid parameter is passed to a
method:
 ArgumentException, ArgumentNullException,
ArgumentOutOfRangeException
When requested operation is not supported
 NotSupportedException
When a method is still not implemented
 NotImplementedException
If no suitable standard exception class is
available
 Create own exception class (inherit Exception)
20

THE TRY-FINALLY STATEMENT
The statement:
Ensures execution of given block in all cases
 When exception is raised or not in the try block
Used for execution of cleaning-up code, e.g. releasing
resources
21
try
{
// Do some work that can cause an exception
}
finally
{
// This block will always execute
}

TRY-FINALLY – EXAMPLE
22
static void TestTryFinally()
{
Console.WriteLine("Code executed before try-finally.");
try
{
string str = Console.ReadLine();
Int32.Parse(str);
Console.WriteLine("Parsing was successful.");
return; // Exit from the current method
}
{
Console.WriteLine("Parsing failed!");
}
finally
{
Console.WriteLine(
"This cleanup code is always executed.");
}
Console.WriteLine(
"This code is after the try-finally block.");
}

EXCEPTIONS – BEST
PRACTICES
catch blocks should begin with the exceptions lowest
in the hierarchy
 And continue with the more general exceptions
 Otherwise a compilation error will occur
Each catch block should handle only these exceptions
which it expects
 If a method is not competent to handle an exception, it should
be left unhandled
 Handling all exceptions disregarding their type is popular bad
practice (anti-pattern)!
23

EXCEPTIONS – BEST PRACTICES
(2)
When raising an exception always pass to the
constructor good explanation message
When throwing an exception always pass a
good description of the problem
 Exception message should explain what causes the
problem and how to solve it
 Good: "Size should be integer in range [1…15]"
 Good: "Invalid state. First call Initialize()"
 Bad: "Unexpected error"
 Bad: "Invalid argument"
24

EXCEPTIONS – BEST
PRACTICES (3)
Exceptions can decrease the application
performance
 Throw exceptions only in situations which are
really exceptional and should be handled
 Do not throw exceptions in the normal program
control flow (e.g. for invalid user input)
CLR could throw exceptions at any time with
no way to predict them
 E.g. System.OutOfMemoryException
25

SUMMARY
Exceptions provide flexible error handling
mechanism in .NET Framework
 Allow errors to be handled at multiple levels
 Each exception handler processes only errors of
particular type (and its child types)
 Other types of errors are processed by some other handlers later
 Unhandled exceptions cause error messages
Try-finally ensures given code block is always
executed (even when an exception is thrown)
26

Exception Handlin g C#.pptx

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