Using a finally Block

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Sử dụng cuối cùng Block Điều quan trọng là hãy nhớ rằng khi một ngoại lệ được ném, nó thay đổi dòng chảy của thực hiện thông qua chương trình. Điều này có nghĩa bạn không thể đảm bảo rằng một tuyên bố sẽ luôn luôn chạy khi phát biểu trước kết thúc

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Nội dung Text: Using a finally Block

Using a finally Block It is important to remember that when an exception is thrown, it changes the flow of execution through the program. This means you can't guarantee that a statement will always run when the previous statement finishes, because the previous statement might throw an exception. Look at the following example. It's very easy to assume the call to reader.Close will always occur. After all, it's right there in the code: TextReader reader = src.OpenText(); string line; while ((line = reader.ReadLine()) != null) { source.Text += line + "\n"; } reader.Close(); Sometimes it's not an issue if one particular statement does not run, but on many occassions it can be a big problem. If the statement releases a resource that was acquired in a previous statement, then failing to execute this statement results in the resource being retained. This example is just such a case: If the call to src.OpenText succeeds, then it acquires a resource (a file handle) and you must ensure that you call reader.Close to release the resource. If you don't, sooner or later you'll run out of file handles and be unable to open more files (if you find file handles too trivial, think of database connections instead). The way to ensure a statement is always run, whether or not an exception has been thrown, is to write that statement inside a finally block. A finally block occurs immediately after a try block, or immediately after the last catch handler after a try block. As long as the program enters the try block associated with a finally block, the finally block will always be run, even if an exception occurs. If an exception is thrown and caught locally, the exception handler executes first, followed by the finally block. If the exception is not caught locally (the common language runtime has to search through the list of calling methods to find a handler), the finally block runs first. In any case, the finally block always executes. The solution to the reader.Close problem is as follows: TextReader reader = null; try { reader = src.OpenText();
string line; while ((line = reader.ReadLine()) != null) { source.Text += line + "\n"; } } finally { if (reader != null) { reader.Close(); } } Even if an exception is thrown, the finally block ensures that the reader.Close statement always executes. You'll see another way to solve this problem in Chapter 13, “Using Garbage Collection and Resource Management.” • If you want to continue to the next chapter Keep Visual Studio 2005 running and turn to Chapter 7. • If you want to exit Visual Studio 2005 now On the File menu, click Exit. If you see a Save dialog box, click Yes.