generate barcode using vb.net 15: Files and Directories in Visual Basic .NET

Paint Quick Response Code in Visual Basic .NET 15: Files and Directories

15: Files and Directories
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Barcode Generation In Visual Basic .NET
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Generics
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When I was in high school, my family sometimes shopped at a local grocery warehouse named Fedmart. Signs on the window clearly stated that there was no connection between Fedmart and the federal government, but people continued to shop there anyway. They had these small $1 frozen cheese pizzas that my mom would buy in bulk for me and my friends, teenage boys who didn t care much about what went down the esophagus. Most of the store stocked the typical grocery products, but there was one aisle near the south border of the store that sold only generic products. Walking into this section was like walking into a black-and-white television; all of the product labels were plain clear or white, with simple black lettering. And they were cheap. They did the job, but just barely. You would never want to run out of name-brand ketchup in the middle of a celebratory barbeque with your friends, and offer up a bottle of generic ketchup as a replacement. Somehow I remember clearly reading the black lettering on the white label of that watery ketchup substitute, about how it met the federal ketchup standards. At that moment I had an epiphany, a sudden realization that would change the way I thought about life in these United States forever: the government has a federal ketchup standard! Sadly, Fedmart closed down before I finished my senior year, leaving a vacuum in the generic ketchup and aluminum foil marketplace. But as a Visual Basic programmer, you can still gain access to generics, through .NET s generics technology. Generics the ability to use placeholders for data types first appeared in Visual Basic 2005 and the related .NET Framework 2.0. This chapter provides you with the specifics on generics.
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What Are Generics
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In .NET, generics is a technology that lets you define data type placeholders within types or methods. Let s say you needed to define a class to track customer data, but you didn t want to enforce a specific format on the customer ID value. Part of your code needs to interact with customer objects using an Integer ID value, while
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another part of the code will use an alphanumeric key for the customer. You might ask, Why don t you just include both types of identifiers as distinct fields in your customer record That wouldn t work because I am trying to come up with a reasonably simple example and answering that question would just distract me. So, here s the numeric version of the class:
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Class CustomerWithNumberID Public ID As Integer Public FullName As String End Class
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Here s the variation that uses a string ID:
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Class CustomerWithStringID Public ID As String Public FullName As String End Class
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Of course, you could define ID as System.Object, and stick anything you wanted in that field. But System.Object is considered weakly typed, and there is nothing to stop you from mixing in Integer and String ID values for different instances in an array of customer objects. What you want is a system that lets you define the class generically, and hold off on specifying the data type of ID until you actually create an instance of the class, or a complete collection of related class instances. With such a system, you could define a general-purpose version of the customer class.
Class CustomerWithSomeID Public ID As <DatatypePlaceholder> Public FullName As String End Class
Later, when it was time to create an instance, you could tell the language which data type to use for the placeholder.
Dim oneCustomer As CustomerWithSomeID(replacing _ <DatatypePlaceholder> with Integer)
This is what generics let you do. Here s the actual Visual Basic syntax that defines the nonspecific customer class:
Class CustomerWithSomeID(Of T) Public ID As T Public FullName As String End Class
The general placeholder, T, appears in a special Of clause, just after the class name. (You don t have to name the placeholder T, but it s become a tradition when presenting sample code using generics.) As a data type, T can be used anywhere within the class definition where you don t want to define the data type upfront. The class, and its ID member, are now ready for instantiation with an actual replacement data type for T. To create a new instance, try this code:
Dim numberCustomer As CustomerWithSomeID(Of Integer)
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