how to generate qr code in asp.net using c# Attributes and Reflection in Visual C#.NET

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Attributes and Reflection
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As well as containing code and data, a .NET program can also contain metadata. Metadata is information about the data that is, information about the types, code, fields, and so on stored along with your program. This chapter explores how some of that metadata is created and used. A lot of the metadata is information that .NET needs in order to understand how your code should be used for example, metadata defines whether a particular method is public or private. But you can also add custom metadata, using attributes. Reflection is the process by which a program can read its own metadata, or metadata from another program. A program is said to reflect on itself or on another program, extracting metadata from the reflected assembly and using that metadata either to inform the user or to modify the program s behavior.
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Attributes
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An attribute is an object that represents data you want to associate with an element in your program. The element to which you attach an attribute is referred to as the target of that attribute. For example, in 12 we saw the XmlIgnore attribute applied to a property:
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[XmlIgnore] public string LastName { get; set; }
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This tells the XML serialization system that we want it to ignore this particular property when converting between XML and objects of this kind. This illustrates an important feature of attributes: they don t do anything on their own. The XmlIgnore attribute contains no code, nor does it cause anything to happen when the relevant property is read or modified. It only has any effect when we use XML serialization, and the only reason it does anything then is because the XML serialization system goes looking for it. Attributes are passive. They are essentially just annotations. For them to be useful, something somewhere needs to look for them.
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Types of Attributes
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Some attributes are supplied as part of the CLR, some by the . NET Framework class libraries, and some by other libraries. In addition, you are free to define custom attributes for your own purposes. Most programmers will use only the attributes provided by existing libraries, though creating your own custom attributes can be a powerful tool when combined with reflection, as described later in this chapter.
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Attribute targets
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If you search through the .NET Framework class libraries, you ll find a great many attributes. Some attributes can be applied to an assembly, others to a class or interface, and some, such as [XmlIgnore], are applied to properties and fields. Most attributes make sense only when applied to certain things the XmlIgnore attribute cannot usefully be applied to a method, for example, because methods cannot be serialized to XML. So each attribute type declares its attribute targets using the AttributeTargets enumeration. Most of the entries in this enumeration are self-explanatory, but since a few are not entirely obvious, Table 17-1 shows a complete list.
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Table 17-1. Possible attribute targets Member name
All Assembly Class Constructor Delegate Enum Event Field GenericParameter Interface Method Module Parameter Property ReturnValue Struct
Attribute may be applied to Any of the following elements: assembly, class, constructor, delegate, enum, event, field, interface, method, module, parameter, property, return value, or struct An assembly A class A constructor A delegate An enumeration An event A field A type parameter for a generic class or method An interface A method A module A parameter of a method A property (both get and set, if implemented) A return value A struct
Applying attributes
You apply most attributes to their targets by placing them in square brackets immediately before the target item. A couple of the target types don t correspond directly to any single source code feature, and so these are handled differently. For example, an assembly is a single compiled .NET executable or library it s everything in a single project so there s no one feature in the source code to which to apply the attribute. Therefore, you can apply assembly attributes at the top of any file. The module attribute target type works the same way.*
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