barcode with vb.net 13: Generics in C#

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13: Generics
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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 advantage
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A Generic Class with Two Type Parameters
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You can declare more than one type parameter in a generic type To specify two or more type parameters, simply use a comma-separated list For example, the following TwoGen class is a variation of the MyGenClass class that has two type parameters:
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// A simple generic class with two type parameters: T and V using System; // TwoGen has two type parameters, T and V class TwoGen<T, V> { T ob1; V ob2;
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// Notice that this constructor uses the two type parameters, T and V public TwoGen(T o1, V o2) { ob1 = o1; ob2 = o2; } public T GetOb1() { return ob1; } public V GetOb2() { return ob2; } } // Demonstrate two generic type parameters class SimpGen { static void Main() { ConsoleWriteLine("Constructing a TwoGen<int, string> object"); TwoGen<int, string> tgObj = new TwoGen<int, string>(1024, "Using two type parameters"); // Obtain and show values int v = tgObjGetOb1(); ConsoleWriteLine("The value of ob1: " + v); string str = tgObjGetOb2(); ConsoleWriteLine("The value of ob2: " + str); } }
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The output from this program is shown here:
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Constructing a TwoGen<int, string> object The value of ob1: 1024 The value of ob2: Using two type parameters
Notice how TwoGen is declared:
class TwoGen<T, V> {
It specifies two type parameters, T and V, separated by a comma Because it has two type parameters, two type arguments must be specified for TwoGen when an object is created, as shown next:
TwoGen<int, string> tgObj = new TwoGen<int, string>(1024, "Using two type parameters");
In this case, int is substituted for T and string is substituted for V Although the two type arguments differ in this example, it is possible for both types to be the same For example, the following line of code is valid:
TwoGen<double, double> x = new TwoGen<double, double>(986, 1024);
In this case, both T and V are of type double Of course, if the type arguments were always the same, then two type parameters would be unnecessary The generics syntax shown in the preceding examples can be generalized Here is the syntax for declaring a generic class: class class-name<type-param-list> { // Here is the syntax for declaring a reference to a generic class and giving it an initial value: class-name<type-arg-list> var-name = new class-name<type-arg-list>(cons-arg-list);
Ask the Expert
Q: A:
You say that classes, structures, methods, interfaces, and delegates can be generic What about properties, operators, indexers, and events Properties, operators, indexers, and events cannot declare type parameters Thus, they cannot be made generic However, they can be used in a generic class and make use of the type parameters defined by that class
13: Generics
Constrained Types
In the preceding examples, the type parameters could be replaced by any type For example, given this declaration:
class MyGenClass<T> {
any type can be specified for T Thus, it is legal to create MyGenClass objects in which T is replaced by int, double, string, FileStream, or any other type Although having no restrictions on the type argument is fine for many purposes, sometimes it is useful to limit the types that can be bound to a type parameter For example, you might want to create a method that operates on the contents of a stream, including a FileStream or MemoryStream This situation seems perfect for generics, but you need some way to ensure that only stream types are used as type arguments You don t want to allow a type argument of int, for example You also need some way to tell the compiler that the methods defined by a stream will be available for use For example, your generic code needs some way to know that it can call the Read( ) method To handle such situations, C# provides constrained types When specifying a type parameter, you can specify a constraint that the type parameter must satisfy This is accomplished through the use of a where clause when specifying the type parameter, as shown here: class class-name<type-param> where type-param : constraints { // Here, constraints is a comma-separated list of constraints C# defines the following types of constraints:
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