06 - Exceptions

Exceptions

Objectives “Unfortunately, errors are a part of life. Some are detected by the compiler, some are detected by the CLR, and the remainder go uncaught. C# and Visual Studio .NET provide many techniques for quickly identifying, dealing with, and eliminating errors. But you have to apply them…”

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Debugging in Visual Studio Exception handling in C# Application design

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Part 1 •

Debugging in Visual Studio…

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Debugging in Visual Studio •

Two main approaches: – print debugging – source-level debugger available in IDE

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The former is understood by all The latter is MUCH more powerful

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Print debugging • •

From a compiled application: – Console.WriteLine or MessageBox.Show If you are running inside Visual Studio: – System.Diagnostics.Debug.WriteLine prints to Output window

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Source-level debugger •

Many powerful features: – pause program at any point – step-by-step execution – view the contents of variables, objects, data structures – show call stack – etc.

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How to use? 1. set a breakpoint to pause program (i.e. to enter "Break" mode) 2. see Debug menu

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Setting breakpoints • Easiest way is to click in left margin of source code

• Other options: – see Debug >> New Breakpoint… – you can break when data changes, or under a condition, or … Microsoft

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Debug menu •

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Step-by-step execution – step into – step over – step out of View locals Immediate window

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Debugging vs. Exception Handling •

Debugging is reactive – it helps you identify and fix errors after the fact – i.e. program has crashed, will crash, or yields incorrect results

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Exception handling is proactive – based on the idea that some errors can be recovered from – i.e. program catches error, fixes problem, and keeps running

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Part 2 •

Exception handling in C#…

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Exceptions •

A signal that an exceptional condition has occurred

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Examples: – divide-by-zero – arithmetic overflow – array access out of bounds – null object reference – file not found – database server unreachable

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Exceptions in .NET • •

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Exceptions are unchecked – i.e. you are not required to document as in Java Exceptions are represented by objects – error codes are "old-fashioned" :-) Different exceptions are denoted by different classes in FCL: System.Exception

System.SystemException

system-defined exceptions

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System.ApplicationException

user-defined exceptions 12

Exception propagation •

What happens if an exception occurs? – object propagates up the call chain, looking for code to handle it – if no such code is found, CLR prints msg & terminates app

call sequence

CLR

Main()

One()

Two()

Three()

exception thrown in Three, call stack is unwound in search for handler… Microsoft

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Example • Example of an unhandled exception: – we try to open a file, but it can't be found… System.IO.FileStream file; file = new System.IO.FileStream("C:\\readme.txt", System.IO.FileMode.Open, System.IO.FileAccess.Read);

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Exception handling in C# •

The basic mechanism is try-catch – try to perform one or more tasks – catch any exceptions that might occur try { // to perform one or more tasks, which might raise an exception… } catch(Exception ex) {

exception

// if anything goes wrong, we end up down here to handle it… }

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Example revisted •

Let's repeat previous example of trying to open a file… try { System.IO.FileStream file; file = new System.IO.FileStream("C:\\readme.txt", System.IO.FileMode.Open, System.IO.FileAccess.Read); . . . file.Close(); }

catch(Exception ex) { System.Diagnostics.Debug.WriteLine(ex.Message); System.Diagnostics.Debug.WriteLine(ex.StackTrace);

MessageBox.Show("Error: " + ex.Message); }

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Semantics of try-catch •

Control flows in one of two ways: – no exception occurs: we skip over Catch block – exception occurs: control transfers immediately to Catch no exception

statement1 fails

try { statement1 statement2 statement3 } catch(Exception ex) { statement4 statement5 } statement6 Microsoft

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Altering the default control flow •

After catching an exception, you might: – consider exception handled & return to caller – rethrow exception if you were unable to handle it – exit application try { . . . } catch(Exception ex) { throw ex; // rethrow exception to caller }

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Part 3 •

Application design…

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Cleanup • •

To ensure proper cleanup, use try-catch-finally pattern Finally block is guaranteed to run after you exit try or catch… System.IO.FileStream

file = null;

try { file = new System.IO.FileStream("C:\\readme.txt", System.IO.FileMode.Open, System.IO.FileAccess.Read); . . . }

catch(Exception ex) { throw ex; } finally { // ensure file is closed if (file != null) file.Close(); } Microsoft

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General exception handling • •

Many different kinds of exceptions can occur Exception kind is determined by object type – use multiple catch blocks, which are applied top-down… try { . . . } catch(System.IO.FileNotFoundException ex) { ... } catch(System.ArithmeticException ex) { ... } catch(Exception ex) { // generic catch-all ... }

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Class design •

Classes should throw exceptions to signal errors… – classes should not attempt to communicate with user (that's the job of the GUI)

public class Employee { . . .

public decimal Salary // salary property… { get { return this.m_Salary; } set { if (value < 0.0M) // invalid salary value! throw new ApplicationException("Invalid Salary!"); else this.m_Salary = value; } }

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App design •

At the very least, application should catch and inform user – easily done within Main method… public static void Main() { try { // to run GUI app… Application.Run( new Form1() ); } catch(Exception ex) { System.Diagnostics.Debug.WriteLine(ex.Message); System.Diagnostics.Debug.WriteLine(ex.StackTrace); MessageBox.Show("Halting due to error: " + ex.Message);

} }

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Summary • •

Unfortunately, errors are a part of programming Some you can recover from, some you cannot

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C# offers reasonable support for exception handling – on par with other programming languages

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Beware! – more subtle than it looks – requires significant effort – don't forget to test! – necessary as computers pervade our lives…

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Lab? •

Work on lab, "OOP and Exceptions"…

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