ASP.NET Web PDF Document Viewer/Editor Control Library

As you may recall from the discussion of Main earlier, the static keyword indicates that we do not need an instance of the containing Program type to be created for this method to run. (We ll be looking at nonstatic methods in the next chapter when we start dealing with objects.) C# follows the C-family convention that the kind of data coming out of the method is specified before the name and the inputs, so next we have double[], indicating that this method returns an array of numbers. Then we have the name, and then in parentheses, the inputs required by this method. In this example there s just one, the filename, but this would be a comma-separated list if more inputs were required. After the method declaration comes the method body the statements that make up the method, enclosed in braces. The code isn t going to be quite the same as what we ve seen so far up until now, we ve converted the text to numbers one at a time immediately before processing them. But this code is going to return an array of numbers, just like File.ReadAllLines returns an array of strings. So our code needs to build up that array. Example 2-17 shows one way of doing this.

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static double[] ReadNumbersFromFile(string fileName) { List<double> numbers = new List<double>(); using (StreamReader file = File.OpenText(fileName)) { while (!file.EndOfStream) { string line = file.ReadLine(); // Skip blank lines if (!string.IsNullOrEmpty(line)) { numbers.Add(double.Parse(line)); } } } return numbers.ToArray(); }

This looks pretty similar to the example while loop we saw earlier, with one addition: we re creating an object that lets us build up a collection of numbers one at a time a List<double> It s similar to an array (a double[]), but an array needs you to know how many items you want up front you can t add more items onto an existing array The advantage of a List<double> is that you can just keep adding new numbers at will That matters here because if you look closely you ll see we ve modified the code to skip over blank lines, which means that we actually don t know how many numbers we re going to get until we ve read the whole file Once you re done adding numbers to a list, you can call its ToArray() method to get an array of the correct size.

Figure 3-16. The list widget dialog in action Listing 3-10 shows how the items are moved between two list widgets. The code shows the slot for moving items from the left list to the right list. First, use the selectedItems().count() method to determine whether there actually is anything to move. The takeItem(int) method

This list class is an example of a collection class NET offers several of these, and they are so extremely useful that s 7, 8, and 9 are related to working with collections Notice the return keyword near the end of Example 2-17 This is how we return the information calculated by our method to whatever code calls the method As well as specifying the value to return, the return keyword causes the current method to exit immediately, and for execution to continue back in the calling method (In methods with a void return type, which do not return any value, you can use the return keyword without an argument to exit the method Or you can just let execution run to the end of the method, and it will return implicitly.

) If you re wondering how the method remembers where it s supposed to go back to, see the sidebar on the next page With the ReadNumbersFromFile method in place, we can now write this sort of code:.

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double[] lapTimes = ReadNumbersFromFile("LapTimes.txt"); double[] fuelLevels = ReadNumbersFromFile("FuelRemainingByLap.txt");

The Mutex class provides a similar style of locking to monitor. The name is short for mutually exclusive indicating that only one thread can acquire the lock at any time. A mutex is significantly more expensive than a monitor because it always gets the OS scheduler involved. And it does that because mutexes work across process boundaries you can create a Mutex object with a name, and if another process in the same Windows login session creates another Mutex object with the same name, that

is used to remove an item from one list widget without having to delete it. This method tells the list widget that you take responsibility for managing the item and removes it from the list widget. You can then add the item to the other list widget using the addItem(QListWidgetItem*) method. This approach enables you to move the items between the list widgets without deleting or creating anything. Listing 3-10. Slot for moving items from the right to the left void ListWidgetDialog::moveLeft() { if( rightList->selectedItems().count() != 1 ) return; QListWidgetItem *item = rightList->takeItem( rightList->currentRow() ); leftList->addItem( item ); }

object refers to the same underlying Windows synchronization object. So to acquire a Mutex, you don t merely have to be the only thread in your application in possession of the lock; you will be the only thread on the whole login session in possession of the lock. (You can even make a global mutex that spans all login sessions, meaning that yours will be the only thread on the entire machine in possession of the lock.) If you create a mutex without a name, it will be local to the process, but it still relies on the OS because a Mutex is essentially a wrapper around a mutex object provided by the Windows kernel.

Monitors are not just for locking, of course they offer coordination facilities through pulsing and waiting. And the .NET Framework offers some slightly more specialized types for coordination too.