C# Exceptions Handling

1. Exceptions HandlingExceptions Handling
Handling Errors during the Program ExecutionHandling Errors during the Program Execution
Svetlin NakovSvetlin Nakov
Telerik CorporationTelerik Corporation
www.telerik.comwww.telerik.com

2. Table of ContentsTable of Contents
1.1. What are Exceptions?What are Exceptions?
2.2. Handling ExceptionsHandling Exceptions
3.3. TheThe System.ExceptionSystem.Exception ClassClass
4.4. Types of Exceptions and theirTypes of Exceptions and their
HierarchyHierarchy
5.5. RaisingRaising ((ThrowingThrowing)) ExceptionsExceptions
6.6. Best PracticesBest Practices
2

3. What are Exceptions?What are Exceptions?
The Paradigm of Exceptions in OOPThe Paradigm of Exceptions in OOP

4. What are Exceptions?What are Exceptions?
 The exceptions in .NET Framework are classicThe exceptions in .NET Framework are classic
implementation of the OOP exception modelimplementation of the OOP exception model
 Deliver powerful mechanism for centralizedDeliver powerful mechanism for centralized
handling of errors and unusual eventshandling of errors and unusual events
 Substitute procedure-oriented approach,Substitute procedure-oriented approach,
in which each function returns error codein which each function returns error code
 Simplify code construction and maintenanceSimplify code construction and maintenance
 Allow the problematic situations to beAllow the problematic situations to be
processed at multiple levelsprocessed at multiple levels
4

5. Handling ExceptionsHandling Exceptions
Catching and Processing ErrorsCatching and Processing Errors

6. Handling ExceptionsHandling Exceptions
 In C# the exceptions can be handled by theIn C# the exceptions can be handled by the
try-catch-finallytry-catch-finally constructionconstruction
 catchcatch blocks can be used multiple times toblocks can be used multiple times to
process different exception typesprocess different exception types
trytry
{{
// Do some work that can raise an exception// Do some work that can raise an exception
}}
catch (SomeException)catch (SomeException)
{{
// Handle the caught exception// Handle the caught exception
}}
6

7. Handling Exceptions – ExampleHandling Exceptions – Example
static void Main()static void Main()
{{
string s = Console.ReadLine();string s = Console.ReadLine();
trytry
{{
Int32.Parse(s);Int32.Parse(s);
Console.WriteLine(Console.WriteLine(
"You entered valid Int32 number {0}.", s);"You entered valid Int32 number {0}.", s);
}}
catchcatch (FormatException)(FormatException)
{{
Console.WriteLine("Invalid integer number!");Console.WriteLine("Invalid integer number!");
}}
catchcatch (OverflowException)(OverflowException)
{{
Console.WriteLine(Console.WriteLine(
"The number is too big to fit in Int32!");"The number is too big to fit in Int32!");
}}
}}
7

8. HandlingHandling
ExceptionsExceptions
Live DemoLive Demo

9. TheThe System.ExceptionSystem.Exception ClassClass
 Exceptions inExceptions in .NET.NET are objectsare objects
 TheThe System.ExceptionSystem.Exception class is base for allclass is base for all
exceptions in CLRexceptions in CLR
 Contains information for the cause of the error orContains information for the cause of the error or
the unusual situationthe unusual situation
 MessageMessage –– text description of the exceptiontext description of the exception
 StackTraceStackTrace –– the snapshot of the stack at thethe snapshot of the stack at the
moment of exception throwingmoment of exception throwing
 InnerExceptionInnerException –– exception caused the currentexception caused the current
exceptionexception ((if anyif any))
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10. Exception Properties – ExampleException Properties – Example
class ExceptionsTestclass ExceptionsTest
{{
public static void CauseFormatException()public static void CauseFormatException()
{{
string s = "an invalid number";string s = "an invalid number";
Int32.Parse(s);Int32.Parse(s);
}}
static void Main()static void Main()
{{
trytry
{{
CauseFormatException();CauseFormatException();
}}
catch (FormatException fe)catch (FormatException fe)
{{
Console.Error.WriteLine("Exception caught: {0}n{1}",Console.Error.WriteLine("Exception caught: {0}n{1}",
fe.Message, fe.StackTrace);fe.Message, fe.StackTrace);
}}
}}
}}
10

11. Exception PropertiesException Properties
 TheThe MessageMessage property gives brief description of theproperty gives brief description of the
problemproblem
 TheThe StackTraceStackTrace property is extremely useful whenproperty is extremely useful when
identifying the reason caused the exceptionidentifying the reason caused the exception
Exception caught: Input string was not in a correctException caught: Input string was not in a correct
format.format.
at System.Number.ParseInt32(String s, NumberStylesat System.Number.ParseInt32(String s, NumberStyles
style, NumberFormatInfo info)style, NumberFormatInfo info)
at System.Int32.Parse(String s)at System.Int32.Parse(String s)
at ExceptionsTest.CauseFormatException() inat ExceptionsTest.CauseFormatException() in
c:consoleapplication1exceptionstest.cs:line 8c:consoleapplication1exceptionstest.cs:line 8
at ExceptionsTest.Main(String[] args) inat ExceptionsTest.Main(String[] args) in
c:consoleapplication1exceptionstest.cs:line 15c:consoleapplication1exceptionstest.cs:line 15
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12. Exception Properties (2)Exception Properties (2)
 File names and line numbers are accessible only if theFile names and line numbers are accessible only if the
compilation was incompilation was in DebugDebug modemode
 When compiled inWhen compiled in ReleaseRelease mode, the information inmode, the information in
the propertythe property StackTraceStackTrace is quite different:is quite different:
Exception caught: Input string was not in a correctException caught: Input string was not in a correct
format.format.
at System.Number.ParseInt32(String s, NumberStylesat System.Number.ParseInt32(String s, NumberStyles
style, NumberFormatInfo info)style, NumberFormatInfo info)
at ExceptionsTest.Main(String[] args)at ExceptionsTest.Main(String[] args)
12

13. Exception PropertiesException Properties
Live DemoLive Demo

14. The Hierarchy ofThe Hierarchy of
ExceptionsExceptions

15.  Exceptions in .NET Framework are organizedExceptions in .NET Framework are organized
in a hierarchyin a hierarchy
Exception HierarchyException Hierarchy
15

16. Types of ExceptionsTypes of Exceptions
 All .NET exceptions inherit fromAll .NET exceptions inherit from System.ExceptionSystem.Exception
 The system exceptions inherit fromThe system exceptions inherit from
System.SystemExceptionSystem.SystemException, e.g., e.g.
 System.ArgumentExceptionSystem.ArgumentException
 System.NullReferenceExceptionSystem.NullReferenceException
 System.OutOfMemoryExceptionSystem.OutOfMemoryException
 System.StackOverflowExceptionSystem.StackOverflowException
 User-defined exceptions should inherit fromUser-defined exceptions should inherit from
System.ApplicationExceptionSystem.ApplicationException
16

17. Handling ExceptionsHandling Exceptions
 When catching an exception of a particular class, allWhen catching an exception of a particular class, all
its inheritors (child exceptions) are caught tooits inheritors (child exceptions) are caught too
 Example:Example:
HandlesHandles ArithmeticExceptionArithmeticException andand its successorsits successors
DivideByZeroExceptionDivideByZeroException andand OverflowExceptionOverflowException
trytry
{{
// Do some works that can raise an exception// Do some works that can raise an exception
}}
catch (System.ArithmeticException)catch (System.ArithmeticException)
{{
// Handle the caught arithmetic exception// Handle the caught arithmetic exception
}}
17

18. Find the Mistake!Find the Mistake!
static void Main(string[] args)static void Main(string[] args)
{{
string s = Console.ReadLine();string s = Console.ReadLine();
trytry
{{
Int32.Parse(s);Int32.Parse(s);
}}
catch (Exception)catch (Exception)
{{
Console.WriteLine("Can not parse the number!");Console.WriteLine("Can not parse the number!");
}}
catch (FormatException)catch (FormatException)
{{
Console.WriteLine("Invalid integer number!");Console.WriteLine("Invalid integer number!");
}}
catch (OverflowException)catch (OverflowException)
{{
Console.WriteLine(Console.WriteLine(
"The number is too big to fit in Int32!");"The number is too big to fit in Int32!");
}}
}}
This should be lastThis should be last
Unreachable codeUnreachable code
Unreachable codeUnreachable code
18

19. Handling All ExceptionsHandling All Exceptions
 All exceptions thrown by .NET managed codeAll exceptions thrown by .NET managed code
inherit theinherit the System.ExceptionSystem.Exception exceptionexception
 Unmanaged code can throw other exceptionsUnmanaged code can throw other exceptions
 For handling all exceptions (even unmanaged)For handling all exceptions (even unmanaged)
use the construction:use the construction:
trytry
{{
// Do some works that can raise any exception// Do some works that can raise any exception
}}
catchcatch
{{
// Handle the caught exception// Handle the caught exception
}}
19

20. Throwing ExceptionsThrowing Exceptions

21. Throwing ExceptionsThrowing Exceptions
 Exceptions are thrown (raised) byExceptions are thrown (raised) by throwthrow
keyword in C#keyword in C#
Used to notify the calling code in case ofUsed to notify the calling code in case of
error or unusual situationerror or unusual situation
 When an exception is thrown:When an exception is thrown:
The program execution stopsThe program execution stops
The exception travels over the stack until aThe exception travels over the stack until a
suitablesuitable catchcatch block is reached to handle itblock is reached to handle it
 Unhandled exceptions display error messageUnhandled exceptions display error message
21

22. How Exceptions Work?How Exceptions Work?
22
Main()Main()
Method 1Method 1
Method 2Method 2
Method NMethod N
2. Method call2. Method call
3. Method call3. Method call
4. Method call4. Method call……
Main()Main()
Method 1Method 1
Method 2Method 2
Method NMethod N
8. Find handler8. Find handler
7. Find handler7. Find handler
6. Find handler6. Find handler……
5. Throw an exception5. Throw an exception
.NET.NET
CLRCLR
1. Execute the
1. Execute theprogram
program
9. Find handler
9. Find handler
10. Display error message
10. Display error message

23. UsingUsing throwthrow KeywordKeyword
 Throwing an exception with error message:Throwing an exception with error message:
 Exceptions can take message and cause:Exceptions can take message and cause:
 NoteNote:: if the original exception is not passed theif the original exception is not passed the
initial cause of the exception is lostinitial cause of the exception is lost
throw new ArgumentException("Invalid amount!");throw new ArgumentException("Invalid amount!");
trytry
{{
Int32.Parse(str);Int32.Parse(str);
}}
catch (FormatException fe)catch (FormatException fe)
{{
throw new ArgumentException("Invalid number", fe);throw new ArgumentException("Invalid number", fe);
}}
23

24. Re-Throwing ExceptionsRe-Throwing Exceptions
 Caught exceptions can be re-thrown again:Caught exceptions can be re-thrown again:
trytry
{{
Int32.Parse(str);Int32.Parse(str);
}}
catch (FormatException fe)catch (FormatException fe)
{{
Console.WriteLine("Parse failed!");Console.WriteLine("Parse failed!");
throw fe; // Re-throw the caught exceptionthrow fe; // Re-throw the caught exception
}}
catch (FormatException)catch (FormatException)
{{
throw; // Re-throws tha last caught exceptionthrow; // Re-throws tha last caught exception
}}
24

25. Throwing Exceptions – ExampleThrowing Exceptions – Example
public static double Sqrt(double value)public static double Sqrt(double value)
{{
if (value < 0)if (value < 0)
throw new System.ArgumentOutOfRangeException(throw new System.ArgumentOutOfRangeException(
"Sqrt for negative numbers is undefined!");"Sqrt for negative numbers is undefined!");
return Math.Sqrt(value);return Math.Sqrt(value);
}}
static void Main()static void Main()
{{
trytry
{{
Sqrt(-1);Sqrt(-1);
}}
catch (ArgumentOutOfRangeException ex)catch (ArgumentOutOfRangeException ex)
{{
Console.Error.WriteLine("Error: " + ex.Message);Console.Error.WriteLine("Error: " + ex.Message);
throw;throw;
}}
}}
25

26. Throwing ExceptionsThrowing Exceptions
Live DemoLive Demo

27. Choosing Exception TypeChoosing Exception Type
27
 When an invalid parameter is passed to a method:When an invalid parameter is passed to a method:
 ArgumentExceptionArgumentException,, ArgumentNullExceptionArgumentNullException,,
ArgumentOutOfRangeExceptionArgumentOutOfRangeException
 When requested operation is not supportedWhen requested operation is not supported
 NotSupportedExceptionNotSupportedException
 When a method is still not implementedWhen a method is still not implemented
 NotImplementedExceptionNotImplementedException
 If no suitable standard exception class is availableIf no suitable standard exception class is available
 Create own exception class (inheritCreate own exception class (inherit ExceptionException))

28. UsingTry-Finally BlocksUsingTry-Finally Blocks

29. TheThe try-finallytry-finally ConstructionConstruction
 The construction:The construction:
 Ensures execution of given block in all casesEnsures execution of given block in all cases
 When exception is raised or not in theWhen exception is raised or not in the trytry blockblock
 Used for execution of cleaning-up code, e.g.Used for execution of cleaning-up code, e.g.
releasing resourcesreleasing resources
trytry
{{
// Do some work that can cause an exception// Do some work that can cause an exception
}}
finallyfinally
{{
// This block will always execute// This block will always execute
}}
29

30. try-finallytry-finally – Example– Example
static void TestTryFinally()static void TestTryFinally()
{{
Console.WriteLine("Code executed before try-finally.");Console.WriteLine("Code executed before try-finally.");
trytry
{{
string str = Console.ReadLine();string str = Console.ReadLine();
Int32.Parse(str);Int32.Parse(str);
Console.WriteLine("Parsing was successful.");Console.WriteLine("Parsing was successful.");
return; // Exit from the current methodreturn; // Exit from the current method
}}
catch (FormatException)catch (FormatException)
{{
Console.WriteLine("Parsing failed!");Console.WriteLine("Parsing failed!");
}}
finallyfinally
{{
Console.WriteLine("This cleanup code is always executed.");Console.WriteLine("This cleanup code is always executed.");
}}
Console.WriteLine("This code is after the try-finally block.");Console.WriteLine("This code is after the try-finally block.");
}}
30

31. Try-FinallyTry-Finally
Live DemoLive Demo

32. Exceptions: Best PracticesExceptions: Best Practices

33. Best PracticesBest Practices
 catchcatch blocks should begin with the exceptionsblocks should begin with the exceptions
lowest in the hierarchy and continue with thelowest in the hierarchy and continue with the
more general exceptionsmore general exceptions
Otherwise a compilation error will occurOtherwise a compilation error will occur
 EachEach catchcatch block should handle only theseblock should handle only these
exceptions which it expectsexceptions which it expects
Handling all exception disregarding their type isHandling all exception disregarding their type is
popular bad practice!popular bad practice!
 When raising an exception always pass to theWhen raising an exception always pass to the
constructor good explanation messageconstructor good explanation message
33

34. BestBest Practices (Practices (22))
 Exceptions can decrease the applicationExceptions can decrease the application
performanceperformance
Throw exceptions only in situations which areThrow exceptions only in situations which are
really exceptional and should be handledreally exceptional and should be handled
Do not throw exceptions in the normal programDo not throw exceptions in the normal program
control flow (e.g.: on invalid user input)control flow (e.g.: on invalid user input)
 Some exceptions can be thrown at any timeSome exceptions can be thrown at any time
with no way to predict them, e.g.:with no way to predict them, e.g.:
System.OutOfMemoryExceptionSystem.OutOfMemoryException
34

35. SummarySummary
 Exceptions provide flexible error handlingExceptions provide flexible error handling
mechanism in .NET Frameworkmechanism in .NET Framework
 Allow errors to be handled at multiple levelsAllow errors to be handled at multiple levels
 Each exception handler processes only errors ofEach exception handler processes only errors of
particular type (and its child types)particular type (and its child types)
 Other types of errors are processed by other handlersOther types of errors are processed by other handlers
 Unhandled exceptions cause error messagesUnhandled exceptions cause error messages
 Try-finally ensures that given code block is alwaysTry-finally ensures that given code block is always
executed (even when an exception is thrown)executed (even when an exception is thrown)
35

36. Exceptions HandlingExceptions Handling
Questions?Questions?
http://academy.telerik.com

37. ExercisesExercises
1.1. Write a program that reads an integer number andWrite a program that reads an integer number and
calculates and prints its square root. If the number iscalculates and prints its square root. If the number is
invalid or negative, print "Invalid number". In allinvalid or negative, print "Invalid number". In all
cases finally print "Good bye". Use try-catch-finally.cases finally print "Good bye". Use try-catch-finally.
2.2. Write a methodWrite a method ReadNumber(int start, intReadNumber(int start, int
end)end) that enters an integer number in given rangethat enters an integer number in given range
[start..end]. If invalid number or non-number text is[start..end]. If invalid number or non-number text is
entered, the method should throw an exception.entered, the method should throw an exception.
Based on this method write a program that entersBased on this method write a program that enters 1010
numbers:numbers:
aa11, a, a22, … a, … a1010, such that 1 < a, such that 1 < a11 < … < a< … < a1010 < 100< 100
37

38. Exercises (2)Exercises (2)
3.3. Write a program that enters file name along with itsWrite a program that enters file name along with its
full file path (e.g.full file path (e.g. C:WINDOWSwin.iniC:WINDOWSwin.ini), reads its), reads its
contents and prints it on the console. Find in MSDNcontents and prints it on the console. Find in MSDN
how to usehow to use System.IO.File.ReadAllText(…)System.IO.File.ReadAllText(…). Be. Be
sure to catch all possible exceptions and print user-sure to catch all possible exceptions and print user-
friendly error messages.friendly error messages.
4.4. Write a program that downloads a file from InternetWrite a program that downloads a file from Internet
(e.g.(e.g. http://www.devbg.org/img/Logo-BASD.jpghttp://www.devbg.org/img/Logo-BASD.jpg))
and stores it the current directory. Find in Googleand stores it the current directory. Find in Google
how to download files in C#. Be sure to catch allhow to download files in C#. Be sure to catch all
exceptions and to free any used resources in theexceptions and to free any used resources in the
finally block.finally block. 38