network adapter driver error code 39 Comparing Delegates for Equality in .NET framework

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Comparing Delegates for Equality
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The Delegate base class overrides Object s virtual Equals method. The MulticastDelegate type inherits Delegate s implementation of Equals. Delegate s implementation of Equals compares two delegate objects to see whether their _target and _methodPtr fields refer to the same object and method. If these two fields match, then Equals returns true; if they don t match, Equals returns false. The following code demonstrates:
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// Construct two delegate objects that refer to the same target/method. Feedback fb1 = new Feedback(FeedbackToConsole); Feedback fb2 = new Feedback(FeedbackToConsole); // Even though fb1 and fb2 refer to two different objects, internally // they both refer to the same callback target/method. Console.WriteLine(fb1.Equals(fb2)); // Displays "True"
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In addition, both the Delegate and MulticastDelegate types provide overloads for the equality (==) and inequality (!=) operators. You can use these operators instead of calling the Equals method. The following code s behavior is identical to that of the preceding code:
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// Construct two delegate objects that refer to the same target/method. Feedback fb1 = new Feedback(FeedbackToConsole); Feedback fb2 = new Feedback(FeedbackToConsole); // Even though fb1 and fb2 refer to two different objects, internally // they both refer to the same callback target/method. Console.WriteLine(fb1 == fb2); // Displays "True"
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Understanding how to compare delegates for equality is important when you try to manipulate delegate chains, a topic I ll talk about next.
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Delegate Chains
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By themselves, delegates are incredibly useful. But add in their support for chaining, and delegates become even more useful. I ve already mentioned that each MulticastDelegate object has a private field, called _prev. This field holds a reference to another MulticastDelegate object; that is, every object of type MulticastDelegate (or any type derived from MulticastDelegate) has a reference to another MulticastDelegate derived object. This field allows delegate objects to be part of a linked list. The Delegate class defines three static methods that you can use to manipulate a linked list chain of delegate objects.
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class System.Delegate { // Combines the chains represented by head and tail; head is returned. // NOTE: head will be the last delegate called. public static Delegate Combine(Delegate tail, Delegate head); // Creates a chain represented by the array of delegates. // NOTE: entry 0 is the head and will be the last delegate called. public static Delegate Combine(Delegate[] delegateArray); // Removes a delegate matching value s Target/Method from the chain. // The new head is returned and will be the last delegate called. public static Delegate Remove(Delegate source, Delegate value);
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When you construct a new delegate object, the object s _prev field is set to null, indicating that no other objects are in the linked list. To combine two delegates into a linked list, you call one of Delegate s static Combine methods:
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Feedback fb1 = new Feedback(FeedbackToConsole); Feedback fb2 = new Feedback(FeedbackToMsgBox); Feedback fbChain = (Feedback) Delegate.Combine(fb1, fb2); // The left side of Figure 17 3 shows what the // chain looks like after this code executes. App appobj = new App(); Feedback fb3 = new Feedback(appobj.FeedbackToStream); fbChain = (Feedback) Delegate.Combine(fbChain, fb3); // The right side of Figure 17 3 shows what the // chain looks like after all the code executes.
Figure 17 3 shows what the internal representation of delegate chains looks like.
Figure 17 3 : Internal representation of delegate chains You ll notice that the Delegate type offers another version of the Combine method that takes an array of Delegate references. Using this version of Com bine, you could rewrite the preceding code as follows:
Feedback[] fbArray[0] fbArray[1] App appobj fbArray[2] fbArray = new Feedback[3]; = new Feedback(FeedbackToConsole); = new Feedback(FeedbackToMsgBox); = new App(); = new Feedback(appobj.FeedbackToStream);
Feedback fbChain = Delegate.Combine(fbArray); // The right side of Figure 17 3 shows what the // chain looks like after all the code executes.
When a delegate is invoked, the compiler generates a call to the delegate type s Invoke method (as discussed earlier in this chapter). To refresh your memory, my earlier example declared a Feedback delegate as follows:
public delegate void Feedback( Object value, Int32 item, Int32 numItems);
This caused the compiler to generate a Feedback class that contains an Invoke method that looks like this (in pseudocode):
class Feedback : MulticastDelegate { public void virtual Invoke( Object value, Int32 item, Int32 numItems) { // If there are any delegates in the chain that // should be called first, call them recursively. if (_prev != null) _prev.Invoke(value, item, numItems); // Call the callback method on the specified target object. _target.methodPtr(value, item, numItems); } }
As you can see, invoking a delegate object causes its previous delegate to be invoked first. When the previous delegate returns, its return value is discarded. After calling its previous delegate, the delegate can then invoke the callback target/method that it wraps. The following code demonstrates.
Feedback fb1 = new Feedback(FeedbackToConsole); Feedback fb2 = new Feedback(FeedbackToMsgBox); Feedback fbChain = (Feedback) Delegate.Combine(fb1, fb2); // At this point, fbChain refers to a delegate that calls // FeedbackToMsgBox, this delegate refers to another delegate that // calls FeedbackToConsole, and this delegate refers to null. // // if // // // Now let s invoke the head delegate, which internally invokes its previous delegate and so on. (fbChain != null) fbChain(null, 0, 10); NOTE: In the preceding line, fbChain will never be null, but checking a delegate reference against null before attempting to invoke it is a very good habit to get into.
So far, I ve shown examples where my delegate type, Feedback, is defined as having a void return value. However, I could have defined my Feedback delegate as follows:
public delegate Int32 Feedback( Object value, Int32 item, Int32 numItems);
If I had, its Invoke method would have internally looked like this (again, in pseudocode):
class Feedback : MulticastDelegate { public Int32 virtual Invoke( Object value, Int32 item, Int32 numItems) { // If there are any delegates in the chain that // should be called first, call them. if (_prev != null) _prev.Invoke(value, item, numItems); // Call the callback method on the specified target object. return _target.methodPtr(value, item, numItems); } }
When the head of the delegate chain is invoked, it calls any previous delegate in the chain. However, notice that the previous delegate s return value is discarded. Your application code will receive the return value only from the delegate that is at the head of the chain (the last callback method called). Note Once constructed, delegate objects are considered to be immutable; that is, delegate objects always have their _prev fields set to null and this never changes. When you Combine a delegate object to a chain, internally, Combine constructs a new delegate object that has the same _target and _methodPtr fields as the source object but the _prev field is set to the old head of the chain. The address of the new delegate object is returned from Combine. This behavior is demonstrated in the following lines of code:
Feedback fb = new Feedback(FeedbackToConsole); Feedback fbChain = (Feedback) Delegate.Combine(fb, fb); // fbChain refers to a chain of two delegate objects. // One of the objects is identical to the object referred // to by fb. The other object was constructed by Combine. // This object s _prev field refers to fb, and Combine // returns the reference to this new object. // Do fb and fbChain refer to the same object "False" Console.WriteLine(Object.ReferenceEquals(fb, fbChain); // Do fb and fbChain refer to the same callback // target/method "True" Console.WriteLine(fb.Equals(fbChain));
Now that you know how to build delegate chains, let s look at how to remove a delegate from a chain. To remove a delegate from a linked list, you call the Delegate type s static Remove method:
Feedback fb1 = new Feedback(FeedbackToConsole); Feedback fb2 = new Feedback(FeedbackToMsgBox); Feedback fbChain = (Feedback) Delegate.Combine(fb1, fb2); // fbChain refers to a chain of two delegates. // Invoke the chain: two methods are called. if (fbChain != null) fbChain(null, 0, 10);
fbChain = (Feedback) Delegate.Remove(fbChain, new Feedback(FeedbackToMsgBox)); // fbChain refers to a chain with one delegate. // Invoke the chain: one method is called. if (fbChain != null) fbChain(null, 0, 10); fbChain = (Feedback) Delegate.Remove(fbChain, new Feedback(FeedbackToConsole)); // fbChain refers to a chain of 0 delegates. (fbChain is null.) // Invoke the chain: 0 methods are called. if (fbChain != null) fbChain(null, 0, 10); // Now you see why I ve been comparing fbChain to null all this time!
This code first builds a chain by constructing two delegate objects and then combining them into a linked list by calling the Combine method. The Remove method is then called. Remove s first parameter refers to the head of the delegate object chain, and the second parameter refers to the delegate object that is to be removed from the chain. I know it seems strange to construct a new 299
delegate object in order to remove it from the chain. To fully understand why this is necessary requires some additional explanation. In the call to Remove, I m constructing a new delegate object. This delegate object has its _target and _methodPtr fields initialized appropriately, and the _prev field is set to null. The Remove method scans the chain (referred to by fbChain), checking whether any of the delegate objects in the chain are equal to the new delegate object. Remember that the overridden Equals method implemented by the Delegate class compares the _target and _methodPtr fields only and ignores the _prev field. If a match is found, the Remove method removes the located delegate object from the chain by fixing up the previous delegate object s _prev field. Re move returns the head of the new chain. If a match is not found, Remove does nothing (no exception is thrown) and returns the same value that was passed for its first parameter. Note that each call to Remove eliminates only one object from the chain, as the following code demonstrates:
Feedback fb = new Feedback(FeedbackToConsole); Feedback fbChain = (Feedback) Delegate.Combine(fb, fb); // fbChain refers to a chain of two delegates. // Invoke the chain: FeedbackToConsole is called twice. if (fbChain != null) fbChain(...); // Remove one of the callbacks from the chain. fbChain = (Feedback) Delegate.Remove(fbChain, fb); // Invoke the chain: FeedbackToConsole is called once. if (fbChain != null) fbChain(...); // Remove one of the callbacks from the chain. fbChain = (Feedback) Delegate.Remove(fbChain, fb); // Invoke the chain: 0 methods are called. if (fbChain != null) fbChain(...);
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