vb.net qr code open source Part II Designing Types in .NET

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Part II Designing Types
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Let s look at a quick example . The Framework Class Library (FCL) defines a generic list algorithm that knows how to manage a set of objects; the data type of these objects is not specified by the generic algorithm . Someone wanting to use the generic list algorithm can specify the exact data type to use with it later . The FCL class that encapsulates the generic list algorithm is called List<T> (pronounced List of Tee), and this class is defined in the System.Collections.Generic namespace . Here is what this class definition looks like (the code is severely abbreviated):
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[Serializable] public class List<T> : IList<T>, ICollection<T>, IEnumerable<T>, IList, ICollection, IEnumerable { public public public public public public public public public public public List(); void Add(T item); Int32 BinarySearch(T item); void Clear(); Boolean Contains(T item); Int32 IndexOf(T item); Boolean Remove(T item); void Sort(); void Sort(IComparer<T> comparer); void Sort(Comparison<T> comparison); T[] ToArray();
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public Int32 Count { get; } public T this[Int32 index] { get; set; } }
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The programmer who defined the generic List class indicates that it works with an unspecified data type by placing the <T> immediately after the class name . When defining a generic type or method, any variables it specifies for types (such as T) are called type parameters . T is a variable name that can be used in source code anywhere a data type can be used . For example, in the List class definition, you see T being used for method parameters (the Add method accepts a parameter of type T) and return values (the ToArray method returns a single-dimension array of type T) . Another example is the indexer method (called this in C#) . The indexer has a get accessor method that returns a value of type T and a set accessor method that accepts a parameter of type T . Since the T variable can be used anywhere that a data type can be specified, it is also possible to use T when defining local variables inside a method or when defining fields inside a type . Note Microsoft s design guidelines state that generic parameter variables should either be
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called T or at least start with an uppercase T (as in TKey and TValue) . The uppercase T stands for type, just as an uppercase I stands for interface (as in IComparable) .
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Now that the generic List<T> type has been defined, other developers can use this generic algorithm by specifying the exact data type they would like the algorithm to operate on . When using a generic type or method, the specified data types are referred to as type arguments . For example, a developer might want to work with the List algorithm by specifying a DateTime type argument . Here is some code that shows this:
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private static void SomeMethod() { // Construct a List that operates on DateTime objects List<DateTime> dtList = new List<DateTime>(); // Add a DateTime object to the list dtList.Add(DateTime.Now); // No boxing // Add another DateTime object to the list dtList.Add(DateTime.MinValue); // No boxing // Attempt to add a String object to the list dtList.Add("1/1/2004"); // Compile-time error // Extract a DateTime object out of the list DateTime dt = dtList[0]; // No cast required }
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Generics provide the following big benefits to developers as exhibited by the code just shown:
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Source code protection The developer using a generic algorithm doesn t need to have access to the algorithm s source code . With C++ templates or Java s generics, however, the algorithm s source code must be available to the developer who is using the algorithm . Type safety When a generic algorithm is used with a specific type, the compiler and the CLR understand this and ensure that only objects compatible with the specified data type are used with the algorithm . Attempting to use an object of an incompatible type will result in either a compiler error or a runtime exception being thrown . In the example, attempting to pass a String object to the Add method results in the compiler issuing an error . Cleaner code Since the compiler enforces type safety, fewer casts are required in your source code, meaning that your code is easier to write and maintain . In the last line of SomeMethod, a developer doesn t need to use a (DateTime) cast to put the result of the indexer (querying element at index 0) into the dt variable . Better performance Before generics, the way to define a generalized algorithm was to define all of its members to work with the Object data type . If you wanted to use the algorithm with value type instances, the CLR had to box the value type instance prior to calling the members of the algorithm . As discussed in 5, Primitive, Reference, and Value Types, boxing causes memory allocations on the managed heap, which causes more frequent garbage collections, which, in turn, hurt an application s
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