ASP.NET Web PDF Document Viewer/Editor Control Library

It is possible to guarantee the order of the threads, but it requires two mutexes and a bigger change Tip

// This class's purpose is to illustrate a difference between // arrays and indexers. Do not use this in real code! class ArrayAndIndexer<T> { public T[] TheArray = new T[100]; public T this[int index] { get { return TheArray[index]; } set { TheArray[index] = value; } } }

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Returns true if the row has updated data that hasn t yet been written to the data source Returns the current index for this row Returns the object containing the underlying row data

You might think that it would make no difference whether we use this class s indexer, or go directly for the array. And some of the time that s true, as it is in this example:

ArrayAndIndexer<int> aai = new ArrayAndIndexer<int>(); aai.TheArray[10] = 42; Console.WriteLine(aai[10]); aai[20] = 99; Console.WriteLine(aai.TheArray[20]);

Mutexes are not intended to guarantee the order of threads; they protect data from being corrupted when several threads try to access the data at the same time. Before you can look at this in detail, you need to understand what the actual problem is. Consider, for example, the expression n += 5. The computer will probably execute this in three steps: 1. Read n from the memory. 2. Add 5 to the value. 3. Write the value back into the memory where n is stored. If two threads try to execute the statement at the same time, the order could end up something like this: 1. Thread A reads the original value of n. 2. Thread A adds 5 to the value. 3. The operating system switches to thread B. 4. Thread B reads the original value of n. 5. Thread B adds 5 to the value. 6. Thread B writes the value back to the memory where n is stored. 7. The operating system switches to thread A. 8. Thread A writes the value back to the memory where n is stored. The result from the execution described previously would be that both threads A and B store the value n+5 in memory and that thread A overwrites the value written by thread B. The result is that the value of n is incorrect (it was supposed to be n+10, but it is n+5).

This swaps freely between using the array and the indexer, and as the output shows, items set through one mechanism are visible through the other:

42 99

However, things are a little different if we make this class store a mutable value type. Here s a very simple modifiable value type:

struct CanChange { public int Number { get; set; } public string Name { get; set; } }

By using a mutex to protect n, you prevent thread B from reaching the value when thread A is working with it, and vice versa. One thread blocks while the other works, so the critical part of the code is executed in series instead of in parallel. By protecting all potentially critical parts of a class from parallel access, the objects can safely be called from multiple threads. The class is said to be thread-safe.

The Number and Name properties both have setters, so this is clearly not an immutable type. This might not seem like a problem we can do more or less exactly the same with this type as we did with int just a moment ago:

Several controls within the Sys.UI.Data namespace provide visual links to your data. You will see examples of using these controls later in the chapter. But before you use them, it s a good idea to understand their interfaces.

ArrayAndIndexer<CanChange> aai = new ArrayAndIndexer<CanChange>(); aai.TheArray[10] = new CanChange { Number = 42 }; Console.WriteLine(aai[10].Number); aai[20] = new CanChange { Number = 99, Name = "My item" }; Console.WriteLine(aai.TheArray[20].Number);

That works fine. The problem arises when we try to modify a property of one of the values already inside the array. We can do it with the array: