barcode printing using c#.net Using the new( ) Constructor Constraint in Visual C#.NET

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Using the new( ) Constructor Constraint
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The new( ) constructor constraint enables you to instantiate an object of a generic type Normally, you cannot create an instance of a generic type parameter However, the new( ) constraint changes this because it requires that a type argument supply a public parameterless constructor This can be the default constructor provided automatically when no explicit constructor is declared or a parameterless constructor explicitly defined by you With the new( ) constraint in place, you can invoke the parameterless constructor to create an object Here is a simple example that illustrates the use of new( ):
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// Demonstrate a new() constructor constraint using System; class MyClass { public MyClass() { // } // } class Test<T> where T : new() { T obj; public Test() { // This works because of the new() constraint obj = new T(); // create a T object } // } class ConsConstraintDemo { static void Main() { Test<MyClass> x = new Test<MyClass>(); } }
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First, notice the declaration of the Test class, shown here:
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class Test<T> where T : new() {
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18:
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Generics
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Because of the new( ) constraint, any type argument must supply a parameterless constructor Next, examine the Test constructor, shown here:
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public Test() { // This works because of the new() constraint obj = new T(); // create a T object }
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PART I
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A new object of type T is created and a reference to it is assigned to obj This statement is valid only because the new( ) constraint ensures that a constructor will be available To prove this, try removing the new( ) constraint and then attempt to recompile the program As you will see, an error will be reported In Main( ), an object of type Test is instantiated, as shown here:
Test<MyClass> x = new Test<MyClass>();
Notice that the type argument is MyClass, and that MyClass defines a parameterless constructor Thus, it is valid for use as a type argument for Test It must be emphasized that it was not necessary for MyClass to explicitly declare a parameterless constructor Its default constructor would also satisfy the constraint However, if a class needs other constructors in addition to a parameterless one, then it would be necessary to also explicitly declare a parameterless version, too There are three important points about using new( ) First, it can be used with other constraints, but it must be the last constraint in the list Second, new( ) allows you to construct an object using only the parameterless constructor, even when other constructors are available In other words, it is not permissible to pass arguments to the constructor of a type parameter Third, you cannot use new( ) in conjunction with a value type constraint, described next
The Reference Type and Value Type Constraints
The next two constraints enable you to indicate that a type argument must be either a reference type or a value type These are useful in the few cases in which the difference between reference and value types is important to generic code Here is the general form of the reference type constraint: where T : class In this form of the where clause, the keyword class specifies that T must be a reference type Thus, an attempt to use a value type, such as int or bool, for T will result in a compilation error Here is the general form of the value type constraint: where T : struct In this case, the keyword struct specifies that T must be a value type (Recall that structures are value types) Thus, an attempt to use a reference type, such as string, for T will result in a compilation error In both cases, when additional constraints are present, class or struct must be the first constraint in the list
Part I:
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