Exception

Exception Handling
• An exception is an error that occurs at
runtime.
• Exception handling streamlines errorhandling by allowing your program to define
a block of code, called an exception
handler, that is executed automatically
when an error occurs.
• It is not necessary to manually check the
success or failure of each specific operation
or method call.

System.Exception Class
• In C#, exceptions are represented by
classes.
• All exception classes must be derived
from the built-in exception class
Exception, which is part of the System
namespace.
• Thus, all exceptions are subclasses of
Exception.
• One very important subclass of Exception
is SystemException.

• SystemException simply defines the top
of the standard exceptions hierarchy.
• The .NET Framework defines several
built-in exceptions that are derived from
SystemException.
• For example, when a division-by-zero is
attempted, a DivideByZeroException
exception is generated.

Fundamentals
• C# exception handling is managed via
four keywords: try, catch, throw , and
finally.
• program statements that you want to
monitor for exceptions are contained
within a try block. If an exception occurs
within the try block, it is thrown. Your
code can catch this exception using catch
and handle it in some rational manner.

Example
class ExcDemo1
{
static void Main()
{
int[] nums = new int[4];
try {
Console.WriteLine("Before exception is
generated.");
for (int i = 0; i < 10; i++)
{
nums[i] = i;

Console.WriteLine("nums[{0}]: {1}", i, nums[i]);
}
}
//specifying exOb is optional.

catch (IndexOutOfRangeException exOb)
{
Console.WriteLine("Index out-of-bounds! “);
}
catch (DivideByZeroException) {
Console.WriteLine("Can't divide by Zero!");
} Console.WriteLine("After catch block.");
} }

Catching All Exceptions
• you might want to catch all exceptions, no
matter the type. To do this, use a catch
clause that specifies no exception type or
variable. It has this general form:
catch {
// handle exceptions
}
• This creates a “catch all” handler that
ensures that all exceptions are caught by
your program.

class ExcDemo5 {
static void Main() {
int[] numer = { 4, 8, 16, 32, 64, 128, 256, 512 };
int[] denom = { 2, 0, 4, 4, 0, 8 };
for(int i=0; i < numer.Length; i++) {
try {
Console.WriteLine(numer[i] + " / " + denom[i]
+ " is " + numer[i]/denom[i]);
}

catch {
Console.WriteLine("Some except
occurred.");} }
}
• There is one point to remember about
using a catch-all catch: It must be the
last catch clause in the catch sequence.

Nesting try Blocks
try { // outer try
for(int i=0; i < numer.Length; i++) {
try { // nested try
Console.WriteLine(numer[i] + " / " +
denom[i] + " is " +
numer[i]/denom[i]);
}
Console.WriteLine("Can't divide by Zero!");
}
} }

catch (IndexOutOfRangeException) {
Console.WriteLine("No matching
element found.");
}

Throwing an Exception
• System-generated exceptions are
automatically thrown by the runtime
system.
• To manually throw an exception, use the
keyword throw .
• Its general form is shown here:
throw exceptOb ;
• The exceptOb must be an object of an
exception class derived from Exception.

class ThrowDemo {
static void Main() {
try {
Console.WriteLine("Before throw.");
throw new DivideByZeroException();
}
Console.WriteLine("Exception caught.");
}
Console.WriteLine("After try/catch
statement."); }}

Using finally
• that method may have opened a file or a
network connection that needs to be
closed. Such types of circumstances are
common in programming,
• and C# provides a convenient way to
handle them: finally.
finally{
//finally code
}

Exception Properties
•

Exception defines several properties. The most interesting are
Message, StackTrace, Source, HelpLink and TargetSite. All are
read-only.

•
•

Message contains a string that describes the nature of the error.
StackTrace contains a string that contains the stack of calls that
lead to the exception.
TargetSite obtains an object that specifies the method that
generated the exception.
The HelpLink here is empty because it was not defined on the
exception.
the Source is the application name.
InnerException-Gets the Exception instance that caused the
current exception.

•
•
•
•

• Error is human made mistake. Error - When the
software deviates from a correct value called error.
• Bug: Error which appears during testing phase.
• Bugs arise from mistakes and errors, made by people,
in either a program’s source code or its design.” Bug When the software does not perform as expected.
• Exception handling is an in built mechanism in .NET
framework to detect and handle run time errors. The C#
language's exception handling features provide a way to
deal with any unexpected or exceptional situations that
arise while a program is running. C# exception handling
is managed via four keywords: try, catch, throw, and
finally.

Race Condition
• A Race Condition occurs when two (or more) threads
attempt to access a shared resource at the same time,
without proper synchronization.
• For example, one thread may be writing a new value to a
variable while another thread is incrementing the
variable’s current value. Without synchronization, the
new value of the variable will depend on the order in
which he threads execute. In situations like this, the two
threads are said to be “racing each other,” with the final
outcome determined by which thread finishes first.
• The solution is prevention: careful programming that
properly synchronizes access to shared resources.

Deadlock
• When developing multithreaded programs, you must be
careful to avoid deadlock and race conditions.
• Deadlock is, as the name implies, a situation in which
one thread is waiting for another thread to do something,
but that other thread is waiting on the first. Thus, both
threads are suspended, waiting for each other, and
neither executes.
• This situation is analogous to two overly polite people
both insisting that the other step through a door first!
• To avoid deadlock, careful programming and thorough
testing is required. In general, if a multithreaded program
occasionally “hangs,” deadlock is the likely cause.

InvalidCastException Class
Mscorlib.dll
• A runtime cast is invalid.
• The exception that is thrown for invalid casting
or explicit conversion.
• An InvalidCastException is generated by the
runtime when a statement tries to cast one
reference type to a reference type that is not
compatible.
• Casts that use the type name in ( ) parentheses
are called explicit casts.

InvalidCastException Class
using System.IO;
using System.Text;
class Program
{
static void Main()
{
StringBuilder reference1 = new StringBuilder();
object reference2 = reference1;
StreamReader reference3 = (StreamReader)reference2;

}
}

output
Unhandled Exception: System.InvalidCastException: Unable to
cast object of type 'System.Text.StringBuilder' to type
'System.IO.StreamReader'. at Program.Main() in …..

Exception

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