17: Reflection, Attributes, and Dynamic Programming in Visual C#

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17: Reflection, Attributes, and Dynamic Programming
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Why Dynamic Binding In addition to reflection, we can define custom types to invoke dynamically Consider using dynamic invocation to retrieve the values of an XML element, for example Rather than using the strongly typed syntax of Listing 1730, using dynamic invocation we could call personFirstName and personLastName
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Listing 1730: The CIL for Attributes in General
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using System; using SystemXmlLinq; // XElement person = XElementParse( @"<Person> <FirstName>Inigo</FirstName> <LastName>Montoya</LastName> </Person>"); ConsoleWriteLine("{0} {1}", personDescendants("FirstName")FirstOrDefault()Value, personDescendants("LastName")FirstOrDefault()Value); //
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Although the code in Listing 1730 is not overly complex, compare it to Listing 1731 an alternative approach that uses a dynamically typed object
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Listing 1731: The CIL for Attributes in General
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using System; // dynamic person = DynamicXmlParse( @"<Person> <FirstName>Inigo</FirstName> <LastName>Montoya</LastName> </Person>"); ConsoleWriteLine("{0} {1}", personFirstName, personLastName); //
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The advantages are clear, but does that mean dynamic programming is preferable to static compilation
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Static Compilation versus Dynamic Programming In Listing 1731, we have the same functionality as in Listing 1730, but there is one very important difference Listing 1730 is entirely statically typed This means that at compile time, all types and their member signatures are verified Method names are required to match, and all parameters are checked for type compatibility This is a key feature of C# and something I have highlighted throughout the book In contrast, Listing 1731 has virtually no statically typed code; the variable person is instead dynamic As a result, there is no compile-time verification that person has a FirstName or LastName property, or any other members, for that matter Furthermore, when coding within an IDE, there is no IntelliSense identifying any members on person The loss of typing would seem to result in a significant decrease in functionality Why is such a possibility even available in C# a functionality that was added in C# 40, in fact Let s examine Listing 1731 again Notice the call to retrieve the "FirstName" element: ElementDescendants("LastName")FirstOrDefault()Value The listing uses a string ("LastName") to identify the element name However, there is no compile-time verification that the string is correct If the casing was inconsistent with the element name or if there was a space, the compile would still succeed, even though a NullReferenceException would occur with the call to the Value property Furthermore, the compiler makes no verification that the "FirstName" element even exists, and if it doesn t, we would also get the NullReferenceException In other words, in spite of all the type-safety advantages, type safety doesn t offer much advantage to accessing the dynamic data stored within the XML element Listing 1731 is no better than Listing 1730 when it comes to compiletime verification of the element retrieval If there is a case mismatch or if the FirstName element didn t exist, there would still be an exception1 However, compare the call to access the first name in Listing 1731 (personFirstName) with the call in Listing 1730 The call is undoubtedly significantly simpler In summary, there are situations where type safety
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1 You cannot use a space in the FirstName property call, but if XML supported spaces in element names, this would be a potential disadvantage, so let s ignore this fact
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17: Reflection, Attributes, and Dynamic Programming
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doesn t and likely can t make certain checks And in such cases, being able to make a dynamic call that is only runtime-verified rather than also compile-time verified is significantly more readable and succinct Obviously, if compile-time verification is possible, statically typed programming is preferred because readable and succinct APIs could accompany it However, in the cases where it isn t effective, C# 40 enables simpler code rather than the purity of type safety
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Implementing a Custom Dynamic Object Listing 1731 included a method call to DynamicXmlParse() that was essentially a factory method call for DynamicXml a custom type rather than one built into the CLR Framework However, DynamicXml doesn t implement a FirstName or LastName property To do so would break the dynamic support for retrieving data from the XML file at execution time, rather than compile-time-based implementation of the XML elements In other words, DynamicXml does not use reflection for accessing its members, but rather it dynamically binds to the values based on the XML content The key to defining a custom dynamic type is implementation of the SystemDynamicIDynamicMetaObjectProvider interface Rather than implementing the interface from scratch, however, the preferred approach is to derive the custom dynamic type from SystemDynamicDynamicObject This provides default implementation for a host of members and allows you to override the ones that don t fit Listing 1732 shows the full implementation
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