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Generic Types Differ Based on Their Type Arguments
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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:
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iOb = strOb; // Wrong!
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Even though both iOb and strOb are of type Gen<T>, they are references to different types because their type arguments differ
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How Generics Improve Type Safety
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At this point, you might be asking yourself the following question Given that the same functionality found in the generic Gen 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 Gen generic The answer is that generics automatically ensure the type safety of all operations involving Gen In the process, generics eliminate the need for you to use casts and type-check code by hand To understand the benefits of generics, first consider the following program that creates a non-generic equivalent of Gen:
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// NonGen is functionally equivalent to Gen but does not use generics using System; class NonGen { object ob; // ob is now of type object // Pass the constructor a reference of type object public NonGen(object o) {
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Part I:
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ob = o; } // Return type object public object GetOb() { return ob; } // Show type of ob public void ShowType() { ConsoleWriteLine("Type of ob is " + obGetType()); } } // Demonstrate the non-generic class class NonGenDemo { static void Main() { NonGen iOb; // Create NonGen object iOb = new NonGen(102); // Show the type of data stored in iOb iObShowType(); // Get the value in iOb // This time, a cast is necessary int v = (int) iObGetOb(); ConsoleWriteLine("value: " + v); ConsoleWriteLine(); // Create another NonGen object and store a string in it NonGen strOb = new NonGen("Non-Generics Test"); // Show the type of data stored in strOb strObShowType(); // Get the value of strOb // Again, notice that a cast is necessary String str = (string) strObGetOb(); ConsoleWriteLine("value: " + str); // This compiles, but is conceptually wrong! iOb = strOb; // The following line results in a runtime exception // v = (int) iObGetOb(); // runtime error! } }
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This program produces the following output:
Type of ob is SystemInt32 value: 102
18:
Generics
Type of ob is SystemString value: Non-Generics Test
As you can see, the output is similar to the previous version of the program There are several things of interest in this version First, notice that NonGen replaces all uses of T with object This makes NonGen able to store any type of object, as can the generic version However, this approach 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 We will begin with this line:
int v = (int) iObGetOb();
PART I
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 NonGen references are the same, and any NonGen reference can refer to any other NonGen 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 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#, that code was not type-safe and its misuse could result in runtime exceptions Generics prevent this from occurring In essence, through generics, what were once runtime errors have become compile-time errors This is a major benefit There is one other point of interest in the NonGen program Notice how the type of the NonGen instance variable ob is obtained by ShowType( ):
ConsoleWriteLine("Type of ob is " + obGetType());
Recall from 11 that object defines several methods that are available to all data types One of these methods is GetType( ), which returns a Type object that describes the type of
Part I:
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