The Stack Definition Using a Data Type Object in Visual C#

Encoder Quick Response Code in Visual C# The Stack Definition Using a Data Type Object
Listing 111 The Stack Definition Using a Data Type Object
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public class Stack { public virtual object Pop(); public virtual void Push(object obj); // }
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Programs frequently use stack type collections to facilitate multiple undo operations For example, Listing 112 uses the stack class for undo operations within a program which simulates the Etch A Sketch game
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Listing 112 Supporting Undo in a Program Similar to the Etch A Sketch Game
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using System; using SystemCollections; class Program { // public void Sketch() { Stack path = new Stack(); Cell currentPosition; ConsoleKeyInfo key; // New with C# 20 do { // Etch in the direction indicated by the // arrow keys that the user enters key = Move(); switch (keyKey)
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{ case ConsoleKeyZ: // Undo the previous Move if (pathCount >= 1) { currentPosition = (Cell)pathPop(); ConsoleSetCursorPosition( currentPositionX, currentPositionY); Undo(); } break ; case case case case ConsoleKeyDownArrow: ConsoleKeyUpArrow: ConsoleKeyLeftArrow: ConsoleKeyRightArrow: // SaveState() currentPosition = new Cell( ConsoleCursorLeft, ConsoleCursorTop); pathPush(currentPosition); break ;
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default: ConsoleBeep(); // New with C#20 break ; } } while (keyKey != ConsoleKeyX); // Use X to quit } } public struct Cell { readonly public int X; readonly public int Y; public Cell(int x, int y) { X = x; Y = y; } }
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The results of Listing 112 appear in Output 111 Using the variable path, which is declared as a SystemCollectionsStack, you save the previous move by passing a custom type, Cell, into the StackPush() method using pathPush(currentPosition) If the user enters a Z (or Ctrl+Z), then you undo the previous move by retrieving it from the stack using a Pop() method, setting the cursor position to be the previous position, and calling Undo() (Note that this code uses some CLR 20-specific console functions as well)
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Output 111
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Although the code is functional, there is a fundamental drawback in the SystemCollectionsStack class As shown in Listing 111, the Stack class collects variables of type object Because every object in the CLR derives from object, Stack provides no validation that the elements you place into it are homogenous or are of the intended type For example, instead of passing currentPosition, you can pass a string in which X and Y are concatenated with a decimal point between them However, the compiler must allow the inconsistent data types because in some scenarios, it is desirable Furthermore, when retrieving the data from the stack using the Pop() method, you must cast the return value to a Cell But if the value returned from the Pop() method is not a Cell type object, an exception is thrown You can test the data type, but splattering such checks builds complexity The fundamental problem with creating classes that can work with multiple data types without generics is that they must use a common base type, generally object data Using value types, such as a struct or integer, with classes that use object exacerbates the problem If you pass a value type to the StackPush() method, for example, the runtime automatically boxes it Similarly, when you retrieve a value type, you need to explicitly unbox the data and cast the object reference you obtain from the Pop() method into a value type While the widening operation (cast to a base class) for a reference type has a negligible performance impact, the box operation for a value type introduces nontrivial overhead To change the Stack class to enforce storage on a particular data type using the preceding C# programming constructs, you must create a specialized stack class, as in Listing 113
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Listing 113 Defining a Specialized Stack Class
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public class CellStack { public virtual Cell Pop(); public virtual void Push(Cell cell); // }
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Because CellStack can store only objects of type Cell, this solution requires a custom implementation of the stack methods, which is less than ideal
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