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The Dynamic Style and COM Automation
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Microsoft Office is programmable through a system called COM automation, which has an adaptable approach to argument counts. Office uses this to good effect. It offers methods which are remarkably flexible because they take an astonishing number of arguments, enabling you to control every conceivable aspect of the operation. The Office APIs are designed to be used from the Visual Basic for Applications (VBA) language, which uses a dynamic idiom, so it doesn t matter if you leave out arguments you re not interested in. Its dynamic method invocation can supply reasonable defaults for any missing values. But this leaves more statically inclined languages with a problem. C# 3.0 requires the number and type of arguments to be known at compile time (even with delegate invocation, the most dynamic form of method invocation available in that language). This means that you don t get to leave out the parts you don t care about you are forced to provide values for every single argument. So although the designers of Microsoft Word intended for you to be able to write code roughly like that shown in Example 18-2:
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var doc = wordApp.Documents.Open("WordFile.docx", ReadOnly:true);
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in C# 3.0 you would have been forced to write the considerably less attractive code shown in Example 18-3.
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object fileName = @"WordFile.docx"; object missing = System.Reflection.Missing.Value; object readOnly = true; var doc = wordApp.Documents.Open(ref fileName, ref missing, ref readOnly, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing, ref missing);
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* Yes, so C# supports variable-length argument lists, but it fakes it. Such methods really have a fixed number of arguments, the last of which happens to be an array. There is only one variable-length Console.WriteLine method, and the compiler is able to determine statically when you use it.
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Not only has C# 3.0 insisted that we supply a value for every argument (using a special this argument intentionally left blank value to signify our intent to provide no particular value), but it has also insisted that we stick precisely to the rules of the type system. Word has chosen about the most general-purpose representation available to ensure maximum flexibility, which is why we see ref in front of every argument it s keeping open the possibility of passing data back out through any of these arguments. It doesn t care that this gives the methods an unusually complex signature, because it just assumes that we ll be using a language whose dynamic method invocation mechanism will automatically perform any necessary conversions at runtime. But if you re using a language with no such mechanism, such as C# 3.0, it s all rather unpleasant. In fact, the way COM automation works is that the target object is ultimately responsible for dealing with defaults, coercion, and so on. The real problem is that C# 3.0 doesn t have any syntax for exploiting this if you want to defer to the COM object, you have to use the dynamic method invocation services provided by reflection, which were described in 17. Unfortunately, doing that from C# 3.0 looks even more unpleasant than Example 18-3. Fortunately, C# 4.0 adds new dynamic features to the language that let us write code like Example 18-2, just as Word intended.
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The dynamic Type
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C# 4.0 introduces a new type called dynamic. In some ways it looks just like any other type such as int, string, or FileStream: you can use it in variable declarations, or function arguments and return types, as Example 18-4 shows. (The method reads a little oddly it s a static method in the sense that it does not relate to any particular object instance. But it s dynamic in the sense that it uses the dynamic type for its parameters and return value.)
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static dynamic AddAnything(dynamic a, dynamic b) { dynamic result = a + b; Console.WriteLine(result); return result; }
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While you can use dynamic almost anywhere you could use any other type name, it has some slightly unusual characteristics, because when you use dynamic, you are really saying I have no idea what sort of thing this is. That means there are some situations where you can t use it you can t derive a class from dynamic, for example, and typeof(dynamic) will not compile. But aside from the places where it would be meaningless, you can use it as you d use any other type.
To see the dynamic behavior in action, we can try passing in a few different things to the AddAnything method from Example 18-4, as Example 18-5 shows.
Console.WriteLine(AddAnything("Hello", "world").GetType().Name); Console.WriteLine(AddAnything(31, 11).GetType().Name); Console.WriteLine(AddAnything("31", 11).GetType().Name); Console.WriteLine(AddAnything(31, 11.5).GetType().Name);
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