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> open System;; > let o = Activator.CreateInstance(Type.GetTypeFromProgID("Word.Application"));; val o : obj > let t = o.GetType();; val t : Type > t.GetProperty("Visible").SetValue(o, (true :> Object), null);; val it : unit = () > let m = t.GetMethod("Quit");; val m : Reflection.MethodInfo > m.GetParameters().Length;; val it : int = 3 > m.GetParameters();; val it : ParameterInfo [] = [|System.Object& SaveChanges {Attributes = In, Optional, HasFieldMarshal; DefaultValue = System.Reflection.Missing; IsIn = true; IsLcid = false; IsOptional = true; IsOut = false; IsRetval = false; Member = Void Quit(System.Object ByRef, System.Object ByRef, System.Object ByRef); MetadataToken = 134223449; Name = "SaveChanges"; ParameterType = System.Object&; Position = 0; RawDefaultValue = System.Reflection.Missing;}; ... more ... |] > m.Invoke(o, [| null; null; null |]);; val it : obj = null Since F# imposes type inference, you cannot use the simple syntax provided by an interpreter. The compiler should know in advance the number and type of arguments of a method and the methods exposed by an object. You must remember that even if fsi.exe allows you to interactively execute F# statements, it still is subjected to the constraints of a compiled language. Since you are creating an instance of a COM component dynamically in this example, the compiler does not know anything about this component. Thus, it can be typed as just System.Object. To obtain the same behavior of an interpreted language, you must resort to the reflection support of the .NET runtime. Using the GetType method, you can obtain an object describing the type of the object o. Then you can obtain a PropertyInfo object describing the Visible property, and you can invoke the SetValue method on it to show the Word main window. The SetValue method is generic; therefore, you have to cast the Boolean value to System.Object to comply with the method signature.
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In a similar way, you can obtain an instance of the MethodInfo class describing the Quit method. Since a method has a signature, you ask for the parameters; there are three of them, and they are optional. Therefore, you can invoke the Quit method by calling the Invoke method and passing the object target of the invocation and an array of arguments that you set to null because arguments are optional. How can the runtime interact with COM components The basic approach is based on the so-called COM callable wrapper (CCW) and the runtime callable wrapper (RCW), as shown in Figure 17-3. The former is a chunk of memory dynamically generated with a layout compatible with the one expected from COM components so that external programs, even legacy Visual Basic 6 applications, can access services implemented as managed components. The latter is more common and creates a .NET type dealing with the COM component, taking care of all the interoperability issues. It is worth noting that although the CCW can always be generated because the .NET runtime has full knowledge about assemblies, the opposite is not always possible. Without IDispatch or type libraries, there is no description of a COM component at run time. Moreover, if a component uses custom marshalling, it cannot be wrapped by an RCW. Fortunately, for the majority of COM components, it is possible to generate the RCW.
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Figure 17-3. The wrappers generated by the CLR to interact with COM components Programming patterns based on event-driven programming are widely adopted, and COM components have a programming pattern to implement callbacks based on the notion of sink. The programming pattern is based on the delegate event model, and the sink is where a listener can register a COM interface that should be invoked by a component to notify an event. The Internet Explorer Web Browser COM component (implemented by shdocvw.dll), for instance, provides a number of events to notify its host about the various events such as loading a page or clicking a hyperlink. The RCW generated by the runtime exposes these events in the form of delegates and takes care of handling all the details required to perform the communication between managed and unmanaged code. Although COM components can be accessed dynamically using .NET reflection, explicitly relying on the ability of the CLR to generate CCW and RCW, it is desirable to use a less verbose
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