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In Figure 21-2, notice that the method s bytes argument (stored in the ESI register) isn t referred to after the CPU instruction at offset 0x00000028 . This means that the Byte array object that the bytes argument refers to can be collected any time after the instruction at offset 0x00000028 executes (assuming that there are no other roots in the application that also refer to this array object) . In other words, as soon as an object becomes unreachable, it is a candidate for collection objects aren t guaranteed to live throughout a method s lifetime . This can have an interesting impact on your application . For example, examine the following code:
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using System; using System.Threading; public static class Program { public static void Main() { // Create a Timer object that knows to call our TimerCallback // method once every 2000 milliseconds. Timer t = new Timer(TimerCallback, null, 0, 2000); // Wait for the user to hit <Enter> Console.ReadLine(); } private static void TimerCallback(Object o) { // Display the date/time when this method got called. Console.WriteLine("In TimerCallback: " + DateTime.Now); // Force a garbage collection to occur for this demo. GC.Collect(); } }
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Compile this code from the command prompt without using any special compiler switches . When you run the resulting executable file, you ll see that the TimerCallback method is called just once! From examining the code above, you d think that the TimerCallback method would get called once every 2,000 milliseconds . After all, a Timer object is created, and the variable t refers to this object . As long as the timer object exists, the timer should keep firing . But you ll notice in the TimerCallback method that I force a garbage collection to occur by calling GC.Collect() . When the collection starts, it first assumes that all objects in the heap are unreachable (garbage); this includes the Timer object . Then, the collector examines the application s roots and sees that Main doesn t use the t variable after the initial assignment to it . Therefore, the application has no variable referring to the Timer object, and the garbage collection reclaims the memory for it; this stops the timer and explains why the TimerCallback method is called just once . Let s say that you re using a debugger to step through Main, and a garbage collection just happens to occur just after t is assigned the address of the new Timer object . Then, let s say that you try to view the object that t refers to by using the debugger s Quick Watch window . What do you think will happen The debugger can t show you the object because it was just garbage collected . This behavior would be considered very unexpected and undesirable by most developers, so Microsoft has come up with a solution . When the JIT compiler compiles the IL for a method into native code, it checks to see if the assembly defining the method was compiled without optimizations and if the process is currently being executed under a debugger . If both are true, the JIT compiler generates the method s internal root table in such a way as to artificially extend the lifetime of all of the variables to the end of the method . In other words, the JIT compiler will trick itself into
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21
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believing that the t variable in Main must live until the end of the method . So, if a garbage collection were to occur, the garbage collector now thinks that t is still a root and that the Timer object that t refers to will continue to be reachable . The Timer object will survive the collection, and the TimerCallback method will get called repeatedly until Console. ReadLine returns and Main exits . This is easy to see . Just run the same executable file under a debugger, and you ll see that the TimerCallback method is called repeatedly! Now, recompile the program from a command prompt, but this time, specify the C# compiler s /debug+ compiler-line switch . When you run the resulting executable file, you ll now see that the TimerCallback method is called repeatedly even if you don t run this program under a debugger! What is happening here Well, when the JIT compiler compiles a method, the JIT compiler looks to see if the assembly that defines the method contains the System.Diagnostics.DebuggableAttribute attribute with its DebuggingModes DisableOptimizations flag set . If the JIT compiler sees this flag set, it also compiles the method, artificially extending the lifetime of all variables until the end of the method . When you specify the /debug+ compiler switch, the C# compiler emits this attribute and flag into the resulting assembly for you . Note, the C# compiler s /optimize+ compiler switch can turn optimizations back on so this compiler switch should not be specified when performing this experiment . The JIT compiler does this to help you with JIT debugging . You may now start your application normally (without a debugger), and if the method is called, the JIT compiler will artificially extend the lifetime of the variables to the end of the method . Later, if you decide to attach a debugger to the process, you can put a breakpoint in a previously compiled method and examine the variables . So now you know how to build a program that works in a debug build but doesn t work correctly when you make a release build! Since no developer wants a program that works only when debugging it, there should be something we can do to the program so that it works all of the time regardless of the type of build . You could try modifying the Main method to this:
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public static // Create // method Timer t = void Main() { a Timer object that knows to call our TimerCallback once every 2000 milliseconds. new Timer(TimerCallback, null, 0, 2000);
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// Wait for the user to hit <Enter> Console.ReadLine(); // Refer to t after ReadLine (this gets optimized away) t = null; }
However, if you compile this (without the /debug+ switch) and run the resulting executable file (not under the debugger), you ll see that the TimerCallback method is still called just once . The problem here is that the JIT compiler is an optimizing compiler, and setting a local
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