.net data matrix barcode generator Debugging Concurrent and Graphical Applications in Visual Basic .NET

Creator DataMatrix in Visual Basic .NET Debugging Concurrent and Graphical Applications

Debugging Concurrent and Graphical Applications
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Although a debugger is a fundamental tool for inspecting applications, it isn t the Holy Grail, and it must be used carefully: be aware that the process interferes with an application s normal execution. The most relevant impact of the debugging process on a running program is the influence on execution timing; and graphical and concurrent programs are much more prevalent these days. Sometimes, a bug disappears while the debugger is being used, due to these changes in execution timing. Debugging and testing concurrent applications can be particularly difficult because using a debugger is guaranteed to alter execution timing. There is no general rule for debugging concurrent applications, but this section briefly discusses how you can use the debugger in these cases. Consider this simple example of a multithreaded application:
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CHAPTER 18 DEBUGGING AND TESTING F# PROGRAMS
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open System open System.Threading let t1 = Thread(fun () -> while true do printf "Thread 1\n" ) let t2 = Thread(fun () -> while true do printf "Thread 2\n" ) t1.Start() t2.Start()
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Note If you run this example in F# Interactive, you must abort the thread explicitly by calling the Abort method,
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right-clicking the F# Interactive window, and choosing Cancel Evaluation. If it doesn t resume, you may have to kill the fsi.exe process that is using the CPU most. This is a common solution when a computation gets out of control during interactive sessions.
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Threads t1 and t2 access the console, which is a shared resource; when you run the program without a debugger attached, the string printed by the two threads appears interleaved on the console. If you set a breakpoint on the two printf statements and start a debugging session, stepping automatically moves from one thread to the other; the output of the program is completely different from that obtained without debugging. This is true also if you disable the breakpoints. The output is even more unbalanced if you set the breakpoint in only one of the two threads. 13 discussed shared-memory multithreaded applications. In such applications, shared objects accessed by different threads are critical resources that may be viewed in the debugger. If the debug of a single thread fails, setting breakpoints in different threads may help you study the dynamic of the application, even if the full interaction of the threads can t be fully simulated. If this approach fails, it may be useful to introduce tests inside the application and use the Debugger type only when a given condition occurs. Channel-based message-passing applications are generally easier to debug than those that rely on shared memory, because you can monitor the communication end points using breakpoints or logging messages. Although careful use of the debugger may help when you re debugging concurrent applications, sometimes external observation is enough to influence a running program. In these cases, tracing through debug output becomes a viable alternative; large systems have different levels of traces to monitor program execution. Graphical applications also present issues when you re debugging. As discussed in 11, a GUI application s event loop is handled by a single thread; if this is blocked, the application s GUI ceases working while it s suspended in the debugger. Consider the following simple application: open System open System.Windows.Forms let f = new Form(Text="Hello world") let b = new Button(Text="Click me!", Dock=DockStyle.Fill) b.Click.Add(fun _ -> b.Text <- "Click me again"
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CHAPTER 18 DEBUGGING AND TESTING F# PROGRAMS
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MessageBox.Show("Hello world") |> ignore ) f.Controls.Add(b) f.Show() Application.Run(f) If you set a breakpoint at the MessageBox statement and debug the application, then when the button is clicked, the debugger suspends execution, and the form stops responding. The text of the button doesn t change until execution resumes, because the thread suspended by the debugger is responsible for handling GUI events, including the paint event that refreshes the button s content and updates the button label. More specifically, event handlers can affect the appearance of a form in two ways: by setting properties of graphical controls and by explicitly drawing using a Graphics object. In the first case, the change isn t noticed until execution resumes; the property change usually asks for a refresh of the control s appearance, which eventually results in a paint event that must be processed by the thread that is suspended in the debugger. In the second case, updates are immediately visible when a statement involving drawing primitives is executed (unless double buffering has been enabled on the window). For example, consider the following program, which displays a window with a number of vertical lines: open System open System.Windows.Forms open System.Drawing let f = new Form(Text="Hello world") f.Paint.Add(fun args -> let g = args.Graphics for i = 0 to f.Width / 10 do g.DrawLine(Pens.Black, i*10, 0, i*10, f.Height) ) f.Show() Application.Run(f) Set a breakpoint at the DrawLine statement and start debugging the application, moving the debugger window in order to make the application form visible. If you continue the execution one statement at a time, you can see the lines appear on the form. In this case, the interaction with the graphical system doesn t trigger an event but interacts directly with the Graphics object by emitting graphic primitives that are rendered immediately. This discussion of debugging graphical applications uses examples based on Windows Forms. The same considerations apply to all event systems where a thread is responsible for event notification. For graphical systems such as WPF, based on the retention of graphic primitives, things work slightly differently, but there are analogous considerations.
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