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
Trang 1Using 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();
Trang 2string 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.”
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