115: Declaring a Nullable Type That Contains a Value Property of Type object in C#

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Listing 115: Declaring a Nullable Type That Contains a Value Property of Type object
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struct Nullable { /// <summary> /// Provides the value when HasValue returns true /// </summary> public object Value{ get; set; }
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Introducing Generic Types
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/// <summary> /// Indicates whether there is a value or whether /// the value is "null" /// </summary> public bool HasValue{ get; set; } }
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Although this option requires only one implementation of a nullable type, the runtime always boxes value types when setting the Value property Furthermore, calls to retrieve data from NullableValue will not be strongly typed, so retrieving the value type will require a cast operation, which is potentially invalid at runtime Neither option is particularly attractive To deal with dilemmas such as this, C# 20 includes the concept of generics In fact, the nullable modifier, , uses generics internally
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Introducing Generic Types
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Generics provide a facility for creating data structures that are specialized to handle specific types when declaring a variable Programmers define these parameterized types so that each variable of a particular generic type has the same internal algorithm, but the types of data and method signatures can vary based on programmer preference To minimize the learning curve for developers, C# designers chose syntax that matched the similar templates concept of C++ In C#, therefore, the syntax for generic classes and structures uses the same angle bracket notation to identify the data types on which the generic declaration specializes
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Using a Generic Class Listing 116 shows how you can specify the actual type used by the generic class You instruct the path variable to use a Cell type by specifying Cell within angle bracket notation in both the instantiation and the declaration expressions In other words, when declaring a variable (path in this case) using a generic data type, C# requires the developer to identify the actual type An example showing the new Stack class appears in Listing 116
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Listing 116: Implementing Undo with a Generic Stack Class
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using System; using SystemCollectionsGeneric; class Program { // public void Sketch() { Stack<Cell> path; path = new Stack<Cell>(); Cell currentPosition; ConsoleKeyInfo key; do { // Etch in the direction indicated by the // arrow keys entered by the user key = Move(); switch (keyKey) { case ConsoleKeyZ: // Undo the previous Move if (pathCount >= 1) { // No cast required currentPosition = pathPop(); ConsoleSetCursorPosition( currentPositionX, currentPositionY); Undo(); } break; case ConsoleKeyDownArrow: case ConsoleKeyUpArrow: case ConsoleKeyLeftArrow: case ConsoleKeyRightArrow: // SaveState() currentPosition = new Cell( ConsoleCursorLeft, ConsoleCursorTop); // Only type Cell allowed in call to Push() pathPush(currentPosition); break; default: ConsoleBeep(); break; } // Generic variable declaration // Generic object instantiation // New with C# 20
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// New with C#20
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Introducing Generic Types
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} while (keyKey != ConsoleKeyX); } } // Use X to quit
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The results of Listing 116 appear in Output 112
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OUTPUT 112:
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In the path declaration shown in Listing 116, you declare and create a new instance of a SystemCollectionsGenericStack<T> class and specify in angle brackets that the data type used for the path variable is Cell As a result, every object added to and retrieved from path is of type Cell In other words, you no longer need to cast the return of pathPop() or ensure that only Cell type objects are added to path in the Push() method Before we examine the generic advantages, the next section introduces the syntax for generic class definitions
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Defining a Simple Generic Class Generics allow you to author algorithms and patterns, and reuse the code for different data types Listing 117 creates a generic Stack<T> class similar to the SystemCollectionsGenericStack<T> class used in the code in Listing 116 You specify a type parameter identifier or type parameter (in this case, T) within angle brackets after the class declaration Instances of
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the generic Stack<T> then collect the type corresponding to the variable declaration without converting the collected item to type object The type parameter T is a placeholder until variable declaration and instantiation, when the compiler requires the code to specify the type parameter In Listing 117, you can see that the type parameter will be used for the internal Items array, the type for the parameter to the Push() method, and the return type for the Pop() method
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