qr code generator c# example The C# Language in C#.NET

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The C# Language
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the invoking object at runtime Thus, even though the type of ob is specified as object in the program s source code, at runtime, the actual type of object being referred to is known This is why the CLR will generate an exception if you try an invalid cast during program execution
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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 Gen class that has two type parameters:
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// A simple generic class with two type parameters: T and V using System; class TwoGen<T, V> { T ob1; V ob2; // Notice that this constructor has parameters of type T and V public TwoGen(T o1, V o2) { ob1 = o1; ob2 = o2; } // Show types of T and V public void showTypes() { ConsoleWriteLine("Type of T is " + typeof(T)); ConsoleWriteLine("Type of V is " + typeof(V)); } public T getob1() { return ob1; } public V GetObj2() { return ob2; } } // Demonstrate two generic type parameters class SimpGen { static void Main() { TwoGen<int, string> tgObj = new TwoGen<int, string>(119, "Alpha Beta Gamma"); // Show the types tgObjshowTypes(); // Obtain and show values int v = tgObjgetob1(); ConsoleWriteLine("value: " + v);
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18:
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Generics
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string str = tgObjGetObj2(); ConsoleWriteLine("value: " + str); }
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PART I PART I PART I
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The output from this program is shown here:
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Type of T is SystemInt32 Type of V is SystemString value: 119 value: Alpha Beta Gamma
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 when a TwoGen object is created, as shown here:
TwoGen<int, string> tgObj = new TwoGen<int, string>(119, "Alpha Beta Gamma");
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<string, string> x = new TwoGen<string, string>("Hello", "Goodbye");
In this case, both T and V would be of type string Of course, if the type arguments were always the same, then two type parameters would be unnecessary
The General Form of a Generic Class
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 generics class: class-name<type-arg-list> var-name = new class-name<type-arg-list>(cons-arg-list);
Constrained Types
In the preceding examples, the type parameters could be replaced by any type For example, given this declaration
class Gen<T> {
any type can be specified for T Thus, it is legal to create Gen objects in which T is replaced by int, double, string, FileStream, or any other type Although having no restrictions on the
Part I:
The C# Language
type argument is fine for many purposes, sometimes it is useful to limit the types that can be used as a type argument 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 You can require that a certain base class be present in a type argument by using a base class constraint This constraint is specified by naming the desired base class There is a variation of this constraint, called a naked type constraint, in which a type parameter (rather than an actual type) specifies the base class This enables you to establish a relationship between two type parameters You can require that one or more interfaces be implemented by a type argument by using an interface constraint This constraint is specified by naming the desired interface You can require that the type argument supply a parameterless constructor This is called a constructor constraint It is specified by new( ) You can specify that a type argument must be a reference type by specifying the reference type constraint: class You can specify that the type argument be a value type by specifying the value type constraint: struct Of these constraints, the base class constraint and the interface constraint are probably the most often used, but all are important Each constraint is examined in the following sections
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