Exception Handling  To use Try blocks to delimit code in which exceptions might occur.  To use Catch blocks to specify exception handlers.  To use the Finally block to release resources.

Uncaught exception Program mostly terminates when there is an uncaught exception ► If debugging in Visual Studio, application pauses and Exception Assistant appears indicating where the exception occurred the exception occurred

Dim n1, n2, r As Integer n1 = num1.Text n1 = num1.Text n2 = num2.Text n2 = num2.Text r = n1 / n2 r = n1 / n2 MessageBox.Show(r) MessageBox.Show(r) Take and assign inputs from user

Fig | Exception Assistant.

► Error Handling is an essential part of any good code. ► Exception  An indication of a problem that occurs during a program ’ s execution  System.Exception is the base class for all exceptions  The coding structure VB.NET uses to deal with such Exceptions is called the Try … Catch structure.

Syntax Try …. Catch Try [ try statements ] Catch ExceptionVariable As ExceptionType [ catch statements ] ‘ Additional Catch block Finally [ finally statements ] End Try

Try Block ► Means “ Try to execute this code “ ► Encloses code that might throw an exception and the code that should not execute if an exception occurs ► Corresponding End Try ► There must be at least one Catch block and/or Finally block immediately after the Try block

Catch Block ► Means "Catch any errors here “ ► Catches and handles an exception  Begins with keyword Catch  Specifies an identifier and exception type ► Example: Catch e As Exception  Executes when exception of proper type matches

Finally Block ► Programs that obtain certain resources must return them explicitly to avoid resource leaks ► Finally block  Optional in a Try statement  Placed after the last Catch block (if there is one)  Executes whether or not an exception is thrown in the corresponding Try block or any of its corresponding Catch blocks  Generally we use it to close an opened file or connection

Termination Model of Exception Handling ► When an exception occurs:  Try block terminates immediately  Program control transfers to first matching Catch block (other Catch block are ignored) ► After exception is handled:  Termination model of exception handling ► Program control does not return to the throw point because the Try block has expired ► Flow of control proceeds to the first statement after the last Catch block  Resumption model of exception handling ► Program control resumes just after throw point

Try n1 = num1.Text n1 = num1.Text n2 = num2.Text n2 = num2.Text r = n1 / n2 r = n1 / n2 MessageBox.Show(r) MessageBox.Show(r) Catch ex As Exception MessageBox.Show(ex.Message) MessageBox.Show(ex.Message) End Try

DivideByZeroTest

► Delimit تحصر او تحدد ► Release يحرر ► Resources المصادر ► Uncaught exception غير مسك استثناء ► Pauses مؤقتا ► Exception Assistant استثناء مساعد ► Essential ضروري ► obtain certain الحصول على بعض ► Termination Model of Exception Handling إنهاء النموذجي لمعالجة استثناء ► Program control transfers to first matching Catch block (other Catch block are ignored)

► نقل برنامج لمراقبة مطابقة أول كتلة الصيد ► Program control does not return to the throw point because the Try block has expired ► مراقبة البرنامج لا عودة الى رمي النقطة لأن حاول عرقلة قد انتهت صلاحيتها ► Resumption model of exception handling فرضية نموذج لمعالجة الاستثناء ► Program control resumes just after throw point ► يستأنف برنامج مراقبة فقط بعد رمي نقطة ► resumes يواصل