barcode with vb.net Generic Types Differ Based on Their Type Arguments in Visual C#.NET

Maker Code 128A in Visual C#.NET Generic Types Differ Based on Their Type Arguments

Generic Types Differ Based on Their Type Arguments
Drawing Code 128 Code Set A In Visual C#
Using Barcode maker for VS .NET Control to generate, create Code 128 Code Set B image in Visual Studio .NET applications.
Code128 Recognizer In C#.NET
Using Barcode scanner for VS .NET Control to read, scan read, scan image in .NET applications.
A key point to understand about generic types is that a reference of one specific version of a generic type is not type-compatible with another version of the same generic type For example, assuming the program just shown, the following line of code is in error and will not compile
Paint Barcode In C#
Using Barcode encoder for .NET framework Control to generate, create barcode image in VS .NET applications.
Recognize Barcode In C#
Using Barcode scanner for Visual Studio .NET Control to read, scan read, scan image in VS .NET applications.
iOb = strOb; // Wrong!
Code 128A Creation In .NET
Using Barcode maker for ASP.NET Control to generate, create Code-128 image in ASP.NET applications.
Drawing ANSI/AIM Code 128 In VS .NET
Using Barcode generator for VS .NET Control to generate, create Code 128C image in .NET applications.
Even though both iOb and strOb are of type MyGenClass<T>, they are references to different types because their type arguments differ
Drawing Code 128C In VB.NET
Using Barcode creator for Visual Studio .NET Control to generate, create USS Code 128 image in .NET applications.
Printing Bar Code In Visual C#.NET
Using Barcode maker for VS .NET Control to generate, create bar code image in .NET framework applications.
Generics Improve Type Safety
Universal Product Code Version A Maker In Visual C#
Using Barcode printer for VS .NET Control to generate, create UPC Symbol image in Visual Studio .NET applications.
Data Matrix 2d Barcode Creator In Visual C#
Using Barcode drawer for .NET framework Control to generate, create ECC200 image in Visual Studio .NET applications.
At this point, you might be asking yourself the following question Given that the same functionality found in the generic MyGenClass class can be achieved without generics, by simply specifying object as the data type and employing the proper casts, what is the benefit of making MyGenClass generic The answer is that generics automatically ensure the type safety of all operations involving MyGenClass In the process, generics eliminate the need for you to use casts and to type-check code by hand To understand the benefits of generics, first consider the following program that creates a non-generic equivalent of MyGenClass called NotGeneric:
Matrix Barcode Creation In C#
Using Barcode generator for .NET Control to generate, create 2D Barcode image in VS .NET applications.
Drawing Code11 In Visual C#.NET
Using Barcode generation for Visual Studio .NET Control to generate, create Code 11 image in VS .NET applications.
// NotGeneric is functionally equivalent to MyGenClass but does // not use generics using System;
Recognizing Code 128 Code Set C In VS .NET
Using Barcode reader for .NET Control to read, scan read, scan image in .NET framework applications.
USS-128 Maker In Objective-C
Using Barcode creator for iPhone Control to generate, create UCC-128 image in iPhone applications.
13: Generics
Data Matrix ECC200 Creation In Objective-C
Using Barcode generation for iPhone Control to generate, create Data Matrix image in iPhone applications.
Code-128 Scanner In VB.NET
Using Barcode reader for .NET Control to read, scan read, scan image in Visual Studio .NET applications.
class NotGeneric { object ob; // ob is now of type object
1D Barcode Encoder In Java
Using Barcode maker for Java Control to generate, create Linear image in Java applications.
Encoding ANSI/AIM Code 128 In Objective-C
Using Barcode creation for iPhone Control to generate, create Code 128 image in iPhone applications.
Use an object reference
GS1 128 Drawer In .NET Framework
Using Barcode printer for ASP.NET Control to generate, create GS1-128 image in ASP.NET applications.
UCC-128 Encoder In Objective-C
Using Barcode printer for iPad Control to generate, create UCC-128 image in iPad applications.
// Pass the constructor a reference of type object public NotGeneric(object o) { ob = o; } // Return type object public object GetOb() { return ob; } } // Demonstrate the non-generic class class NonGenDemo { static void Main() { NotGeneric iOb; // Create NotGeneric object iOb = new NotGeneric(88); // Get the value in iOb // This time, a cast is necessary int v = (int) iObGetOb(); Cast needed ConsoleWriteLine("iOb is an instance of NotGeneric\n" + "Therefore, the value returned by GetOb() " + "is object\nIt must be cast to int: " + v + "\n"); // Create another NotGeneric object and store a string in it NotGeneric strOb = new NotGeneric("Non-Generic class"); // Get the value of strOb // Again, notice that a cast is necessary String str = (string) strObGetOb(); Cast needed ConsoleWriteLine("strOb is an instance of NotGeneric\n" + "Therefore, the value returned by GetOb() " + "is also object\nIt must be cast to string: " + str + "\n"); // This compiles, but is conceptually wrong! iOb = strOb; // The following line results in a runtime exception Runtime type mismatch // v = (int) iObGetOb(); // runtime error! } }
C# 30: A Beginner s Guide
This program produces the following output:
iOb is an instance of NotGeneric Therefore, the value returned by GetOb() is object It must be cast to int: 88 strOb is an instance of NotGeneric Therefore, the value returned by GetOb() is also object It must be cast to string: Non-Generic class
There are several things of interest in this version First, notice that NotGeneric replaces all uses of T with object This makes NotGeneric able to store any type of object, as can the generic version However, this is bad for two reasons First, explicit casts must be employed to retrieve the stored data Second, many kinds of type mismatch errors cannot be found until runtime Let s look closely at each problem First, notice this line:
int v = (int) iObGetOb();
Because the return type of GetOb( ) is now object, the cast to int is necessary to enable the value returned by GetOb( ) to be unboxed and stored in v If you remove the cast, the program will not compile In the generic version of the program, this cast was not needed because int was specified as a type argument when iOb was constructed In the non-generic version, the cast must be employed This is not only an inconvenience, but a potential source of error Now, consider the following sequence from near the end of the program
// This compiles, but is conceptually wrong! iOb = strOb; // The following line results in a runtime exception // v = (int) iObGetOb(); // runtime error!
Here, strOb is assigned to iOb However, strOb refers to an object that contains a string, not an integer This assignment is syntactically valid because all NotGeneric references are of the same type Thus, any NotGeneric reference can refer to any NotGeneric object However, the statement is semantically wrong, as the commented-out line shows In that line, the return type of GetOb( ) is cast to int and then an attempt is made to assign this value to v The trouble is that iOb now refers to an object that stores a string, not an int Unfortunately, without the use of generics, the compiler won t catch this error Instead, a runtime exception will occur when the cast to int is attempted To see this for yourself, try removing the comment symbol from the start of the line, and then compiling and running the program A runtime error will occur The preceding sequence can t occur when generics are used If this sequence were attempted in the generic version of the program, the compiler would catch it and report an error, thus preventing a serious bug that results in a runtime exception The ability to create type-safe code in which type-mismatch errors are caught at compile time is a key advantage of generics Although using object references to create generic code has always been possible in C#,
Copyright © OnBarcode.com . All rights reserved.