Writing Checked Expressions in .NET framework

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Writing Checked Expressions
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You can also use the checked and unchecked keywords to control over ow checking on integer expressions by preceding just the individual parenthesized expression with the checked or unchecked keyword, as shown in this example:
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int wontThrow = unchecked(int.MaxValue + 1); int willThrow = checked(int.MaxValue + 1);
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The compound operators (such as += and -=) and the increment (++) and decrement (--) operators are arithmetic operators and can be controlled by using the checked and unchecked keywords. Remember, x += y; is the same as x = x + y;. Important You cannot use the checked and unchecked keywords to control oating-point (noninteger) arithmetic. The checked and unchecked keywords apply only to integer arithmetic using data types such as int and long. Floating-point arithmetic never throws Over owException not even when you divide by 0.0. (The .NET Framework has a representation for in nity.) In the following exercise, you will see how to perform checked arithmetic when using Visual Studio 2008.
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Use checked expressions
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1. Return to Visual Studio 2008. 2. On the Debug menu, click Start Without Debugging. You will now attempt to multiply two large values. 3. Type 9876543 in the left operand text box, type 9876543 in the right operand text box, select the Multiplication option, and then click Calculate. The value 1195595903 appears in the Result text box on the form. This is a negative value, which cannot possibly be correct. This value is the result of a multiplication operation that silently over owed the 32-bit limit of the int type.
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Part I
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Introducing Microsoft Visual C# and Microsoft Visual Studio 2008
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4. Click Quit, and return to the Visual Studio 2008 programming environment. 5. In the Code and Text Editor window displaying Window1.xaml.cs, locate the multiplyValues method. It looks like this:
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private int multiplyValues(int leftHandSide, int rightHandSide) { expression.Text = leftHandSide.ToString() + * + rightHandSide.ToString(); return leftHandSide * rightHandSide; }
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The return statement contains the multiplication operation that is silently over owing. 6. Edit the return statement so that the return value is checked, like this:
return checked(leftHandSide * rightHandSide);
The multiplication is now checked and will throw an Over owException rather than silently returning the wrong answer. 7. Locate the calculateClick method. 8. Add the following catch handler immediately after the existing FormatException catch handler in the calculateClick method:
catch (OverflowException oEx) { result.Text = oEx.Message; }
Tip The logic of this catch handler is the same as that for the FormatException catch
handler. However, it is still worth keeping these handlers separate rather than simply writing a generic Exception catch handler because you might decide to handle these exceptions differently in the future.
9. On the Debug menu, click Start Without Debugging to build and run the application. 10. Type 9876543 in the left operand text box, type 9876543 in the right operand text box, select the Multiplication option, and then click Calculate. The second catch handler successfully catches the Over owException and displays the message Arithmetic operation resulted in an over ow in the Result text box. 11. Click Quit to return to the Visual Studio 2008 programming environment.
Throwing Exceptions
Suppose you are implementing a method called monthName that accepts a single int argument and returns the name of the corresponding month. For example, monthName(1) returns January , monthName(2) returns February , and so on. The question is: What should
6
Managing Errors and Exceptions
the method return when the integer argument is less than 1 or greater than 12 The best answer is that the method shouldn t return anything at all; it should throw an exception. The .NET Framework class libraries contain lots of exception classes speci cally designed for situations such as this. Most of the time, you will nd that one of these classes describes your exceptional condition. (If not, you can easily create your own exception class, but you need to know a bit more about the C# language before you can do that.) In this case, the existing .NET Framework ArgumentOutOfRangeException class is just right. You can throw an exception by using the throw statement, as shown in the following example:
public static string monthName(int month) { switch (month) { case 1 : return January ; case 2 : return February ; ... case 12 : return December ; default : throw new ArgumentOutOfRangeException( Bad month ); } }
The throw statement needs an exception object to throw. This object contains the details of the exception, including any error messages. This example uses an expression that creates a new ArgumentOutOfRangeException object. The object is initialized with a string that will populate its Message property by using a constructor. Constructors are covered in detail in 7, Creating and Managing Classes and Objects. In the following exercises, you will add to the MathsOperators project code for throwing an exception.
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