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FIguRE 17-2 A variable that refers to a delegate to a static method and a variable that refers to a delegate to
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The Delegate class defines two read-only public instance properties: Target and Method . Given a reference to a delegate object, you can query these properties . The Target property returns a reference to the object that will be operated on if the method is called back . Basically, the Target property returns the value stored in the private _target field . If the delegate object wraps a static method, Target returns null . The Method property returns a reference to a System.Reflection.MethodInfo object that identifies the callback method . Basically, the Method property has an internal mechanism that converts the value in the private _methodPtr field to a MethodInfo object and returns it . You could use this information in several ways . For example, you could check to see if a delegate object refers to an instance method of a specific type:
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Boolean DelegateRefersToInstanceMethodOfType(MulticastDelegate d, Type type) { return((d.Target != null) && d.Target.GetType() == type); }
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You could also write code to check if the callback method has a specific name (such as FeedbackToMsgBox):
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Boolean DelegateRefersToMethodOfName(MulticastDelegate d, String methodName) { return(d.Method.Name == methodName); }
There are many other potential uses of these properties . Now that you know how delegate objects are constructed and what their internal structure looks like, let s talk about how the callback method is invoked . For convenience, I ve repeated the code for the Counter method here:
private static void Counter(Int32 from, Int32 to, Feedback fb) { for (Int32 val = from; val <= to; val++) { // If any callbacks are specified, call them if (fb != null) fb(val); } }
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Look at the line of code just below the comment . The if statement first checks to see if fb is not null . If fb is not null, on the next line, you see the code that invokes the callback method . The null check is required because fb is really just a variable that can refer to a Feedback delegate object; it could also be null . It might seem as if I m calling a function named fb and passing it one parameter (val) . However, there is no function called fb . Again, because it knows that fb is a variable that refers to a delegate object, the compiler generates code to call the delegate object s Invoke method . In other words, the compiler sees this:
fb(val);
But the compiler generates code as though the source code said this:
fb.Invoke(val);
You can verify that the compiler produces code to call the delegate type s Invoke method by using ILDasm .exe to examine the Intermediate Language (IL) code created for the Counter method . Here is the IL for the Counter method . The instruction at IL_0009 in the figure indicates the call to Feedback s Invoke method .
.method private hidebysig static void Counter(int32 from, int32 'to', class Feedback fb) cil managed
{ // Code size 23 (0x17) .maxstack 2 .locals init (int32 V_0) IL_0000: ldarg.0 IL_0001: stloc.0 IL_0002: br.s IL_0012 IL_0004: ldarg.2 IL_0005: brfalse.s IL_000e IL_0007: ldarg.2 IL_0008: ldloc.0 IL_0009: callvirt instance void Feedback::Invoke(int32) IL_000e: ldloc.0 IL_000f: ldc.i4.1 IL_0010: add IL_0011: stloc.0 IL_0012: ldloc.0 IL_0013: ldarg.1 IL_0014: ble.s IL_0004 IL_0016: ret } // end of method Program::Counter
In fact, you could modify the Counter method to call Invoke explicitly, as shown here:
private static void Counter(Int32 from, Int32 to, Feedback fb) { for (Int32 val = from; val <= to; val++) { // If any callbacks are specified, call them if (fb != null) fb.Invoke(val); } }
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