Throwing and Catching Exceptions in C#

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Throwing and Catching Exceptions
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The process of creating a new error object when a particular condition occurs is called throwing an exception For example, upon dividing by zero, the CLR causes an exception object (of type SystemDivideByZeroException) to be thrown and then looks for a handler (code that catches the exception) In the following example, there is no handler
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to catch the error, so the CLR displays the dialog box shown in Figure 4-2 to the user of the program, and then causes the program to end
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public class DivideByZero { public static void Main() { int x = 0; int y = 10; int z = y / x; SystemConsoleWriteLine ("We'll never get here in the code"); } }
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Coding to Handle Errors and Exceptions
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There are three types of code blocks that you can use to test, catch, and handle runtime errors: try block the code This tells the CLR that we know an exception might occur in
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catch block This is the block of code that tells the CLR what to do if a specific exception has occurred finally block This is the last piece of code to be executed, whether an exception occurred or not Before we examine each of these blocks in detail, take a moment to look at the following sequence of events that will take us through the steps of how the exception handling flows: 1 Program execution enters a try block Each line of code is executed If no error occurs, control is transferred to the nearest finally block (Step 4 in this list) If no finally block exists, execution continues on the instruction following the last catch block 2 If an error occurs, the CLR creates an object to represent the error and transfers execution control to the catch blocks Each catch block is examined, in
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Figure 4-2 The dialog box presented to the user before the program closes
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4: Strings, Exceptions, and Events
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order of appearance If a catch block is found that deals with the error object, the code within that block is executed and control is then transferred to the finally block (step 4 in this list) 3 If no catch block is found that handles the error object, the CLR passes the error object to the caller of the current method The process described in step 2 continues in the caller method Again, if no catch block is found, the error is passed back until it reaches the Main() method Once in Main(), the error is either caught or it causes the program to terminate prematurely and it displays a dialog box like the one shown earlier in Figure 4-2 4 The finally block is executed Notice that we do not mention any conditions on this block of code executing This is because you are guaranteed (unless some abnormal abort like a power failure or a kernel abort occurs) that the finally block of code will run, whether there was an error or not In the following sections, we will examine the use of the try and catch blocks and then look at the benefits of coding a good finally block We will also explore the SystemException class and its children in order to understand the types of exceptions that may occur PART I
The try catch Block
If you suspect that your code may cause an exception to be thrown, you can first place that code within a try block True to its name, it tells the compiler to try the code and, if an error occurs, exit from the block and check for a catch block that handles the error If we recode our DivideByZero example and use a try block, we can handle the error without the user getting involved at all
public class DivideByZeroHandled { public static void Main() { try { int x = 0; int y = 10; int z = y / x; SystemConsoleWriteLine ("We'll never get here in the code"); } catch (SystemException e) { SystemConsoleWriteLine ("We caught an error!"); // write code to handle the error here } SystemConsoleWriteLine ("We will now get to the end of the code!"); } }
Here we ve told the CLR to try to execute the code in the try block Of course, we know that the division by zero will cause an error The CLR will then create an object to represent the error Later in this chapter, we will deal with the types of objects that it
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