14: When a Class Isn t a Class The Interface and the Structure in Visual C#

Create Code 128 Code Set C in Visual C# 14: When a Class Isn t a Class The Interface and the Structure

Structures and classes do have one striking similarity: They both derive from Object In fact, all classes and structures, whether they say so or not, derive from Object This fact unifies the different variable types into one all-encompassing class hierarchy

The similarity between structures and simple value types is more than skin deep In fact, a simple value type is a structure For example, int is another name for the structure Int32, double is another name for the structure Double, and so forth Table 14-1 shows the full list of types and their corresponding struct names

Table 14-1

struct Name Boolean Byte SByte Char Decimal Double Single Int32 UInt32 Int64 UInt64 Object Int16 UInt16

The string type is a reference type, not a value type, so no struct exists for it Instead, string corresponds to class String Recall, however, the string is a special C# animal, as it has been granted some unusual struct-like properties 9 discusses the string type in detail

An int is another name for Int32 (a kind of alias, really) Because all structs derive from Object, int must derive from Object as well This leads to some fascinating results, as the following program demonstrates:

// TypeUnification - demonstrate how int and Int32 // are actually the same thing and // how they derive from Object using System; namespace TypeUnification { public class Program { public static void Main(string[] args) { // create an int and initialize it to zero int i = new int(); // yes, you can do this // assign it a value and output it via the // IFormattable interface that Int32 implements i = 1; OutputFunction(i); // the constant 2 also implements IFormattable OutputFunction(2); // in fact, you can call a method of a constant ConsoleWriteLine( Output directly = {0} , 3ToString()); // this can be truly useful; you can pick an int out of a list: ConsoleWriteLine( \nPick the integers out of a list ); object[] objects = new object[5]; objects[0] = this is a string ; objects[1] = 2; objects[2] = new Program(); objects[3] = 4; objects[4] = 55; for(int index = 0; index < objectsLength; index++) { if (objects[index] is int) { int n = (int)objects[index]; ConsoleWriteLine( the {0}th element is a {1} , index, n);

} } // type unity allows you to display value and // reference types without differentiating them ConsoleWriteLine( \nDisplay all the objects in the list ); int nCount = 0; foreach(object o in objects) { ConsoleWriteLine( Objects[{0}] is <{1}> , nCount++, oToString()); // all objects implement IFormattable } // wait for user to acknowledge the results ConsoleWriteLine( Press Enter to terminate ); ConsoleRead(); } // OutputFunction - outputs any object that implements ToString() // public static void OutputFunction(IFormattable id) { ConsoleWriteLine( Value from OutputFunction = {0} , idToString()); } // ToString - provide a simple string function for the Program class override public string ToString() { return TypeUnification Program ; } } }

This program puts the Int32 struct through its paces Main() begins by creating an int object i Main() uses the Int32() default constructor (or you could say the int() constructor) to initialize i to zero The program continues by assigning a value to i Admittedly, this differs slightly from the format you would use for a structure that you may create

Main() passes the variable i to OutputFunction(), which is declared to accept an object that implements the IFormattable interface The IFormattable interface is similar to the IDisplayable interface that I define in other programs the only method in IFormattable is ToString All classes and structures inherit the IFormattable interface from Object, so all objects, whether reference or value types, implement ToString() OutputFunction() tells the IFormattable object to display itself the Int32 variable has no problem because it has its own ToString() method This is demonstrated even more graphically in the call OutputFunction(2) Being of type Int32, the constant 2 also implements IFormattable Finally, just to shove your nose in it, Main() invokes 3ToString() directly The output from this first section of Main() is as follows: