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Printing QR-Code in .NET framework Part IV

Part IV
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using Reflection to Build a Dynamically Extensible Application
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As you know, metadata is stored in a bunch of tables . When you build an assembly or a module, the compiler that you re using creates a type definition table, a field definition table, a method definition table, and so on . The System.Reflection namespace contains several types that allow you to write code that reflects over (or parses) these metadata tables . In effect, the types in this namespace offer an object model over the metadata contained in an assembly or a module . Using these object model types, you can easily enumerate all of the types in a type definition metadata table . Then for each type, you can obtain its base type, the interfaces it implements, and the flags that are associated with the type . Additional types in the System.Reflection namespace allow you to query the type s fields, methods, properties, and events by parsing the corresponding metadata tables . You can also discover any custom attributes (covered in 18, Custom Attributes ) that have been applied to any of the metadata entities . There are even classes that let you determine referenced assemblies and methods that return the IL byte stream for a method . With all of this information, you could easily build a tool very similar to Microsoft s ILDasm .exe . Note You should be aware that some of the reflection types and some of the members defined
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by these types are designed specifically for use by developers who are producing compilers for the CLR . Application developers don t typically use these types and members . The Framework Class Library (FCL) documentation doesn t explicitly point out which of these types and members are for compiler developers rather than application developers, but if you realize that not all reflection types and their members are for everyone, the documentation can be less confusing .
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In reality, very few applications will have the need to use the reflection types . Reflection is typically used by class libraries that need to understand a type s definition in order to provide some rich functionality . For example, the FCL s serialization mechanism (discussed in 24, Runtime Serialization ) uses reflection to determine what fields a type defines . The serialization formatter can then obtain the values of these fields and write them into a byte stream that is used for sending across the Internet, saving to a file, or copying to the clipboard . Similarly, Visual Studio s designers use reflection to determine which properties should be shown to developers when laying out controls on their Web Forms or Windows Forms at design time . Reflection is also used when an application needs to load a specific type from a specific assembly at runtime to accomplish some task . For example, an application might ask the user to provide the name of an assembly and a type . The application could then explicitly load the assembly, construct an instance of the type, and call methods defined in the type . This usage is conceptually similar to calling Win32 s LoadLibrary and GetProcAddress functions .
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23 Assembly Loading and Reflection
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Binding to types and calling methods in this way is frequently referred to as late binding . (Early binding is when the types and methods used by an application are determined at compile time .)
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Reflection is an extremely powerful mechanism because it allows you to discover and use types and members at runtime that you did not know about at compile time . This power does come with two main drawbacks:
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Reflection prevents type safety at compile time . Since reflection uses strings heavily, you lose type safety at compile time . For example, if you call Type.GetType("Jef"); to ask reflection to find a type called Jef in an assembly that has a type called Jeff, the code compiles but produces an error at runtime because you accidentally misspelled the type name passed as the argument . Reflection is slow . When using reflection, the names of types and their members are not known at compile time; you discover them at runtime by using a string name to identify each type and member . This means that reflection is constantly performing string searches as the types in the System.Reflection namespace scan through an assembly s metadata . Often, the string searches are case-insensitive comparisons, which can slow this down even more .
Invoking a member by using reflection will also hurt performance . When using reflection to invoke a method, you must first package the arguments into an array; internally, reflection must unpack these on to the thread s stack . Also, the CLR must check that the arguments are of the correct data type before invoking a method . Finally, the CLR ensures that the caller has the proper security permission to access the member being invoked . For all of these reasons, it s best to avoid using reflection to access a field or invoke a method/property . If you re writing an application that will dynamically discover and construct type instances, you should take one of the following approaches:
Have the types derive from a base type that is known at compile time . At runtime, construct an instance of the derived type, place the reference in a variable that is of the base type (by way of a cast), and call virtual methods defined by the base type . Have the type implement an interface that is known at compile time . At runtime, construct an instance of the type, place the reference in a variable that is of the interface type (by way of a cast), and call the methods defined by the interface .
I tend to prefer using the interface technique over the base type technique because the base type technique doesn t allow the developer to choose the base type that works best in a particular situation . Although the base type technique works better in versioning scenarios since you could always add a member to the base type and the derived types just inherit it;
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