vb.net generate barcode image Part II Designing Types in Visual C#

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Part II Designing Types
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The is operator is typically used as follows:
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if (o is Employee) { Employee e = (Employee) o; // Use e within the remainder of the 'if' statement. }
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In this code, the CLR is actually checking the object s type twice: The is operator first checks to see if o is compatible with the Employee type . If it is, inside the if statement, the CLR again verifies that o refers to an Employee when performing the cast . The CLR s type checking improves security, but it certainly comes at a performance cost, because the CLR must determine the actual type of the object referred to by the variable (o), and then the CLR must walk the inheritance hierarchy, checking each base type against the specified type (Employee) . Because this programming paradigm is quite common, C# offers a way to simplify this code and improve its performance by providing an as operator:
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Employee e = o as Employee; if (e != null) { // Use e within the 'if' statement. }
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In this code, the CLR checks if o is compatible with the Employee type, and if it is, as returns a non-null reference to the same object . If o is not compatible with the Employee type, the as operator returns null . Notice that the as operator causes the CLR to verify an object s type just once . The if statement simply checks whether e is null; this check can be performed faster than verifying an object s type . The as operator works just as casting does except that the as operator will never throw an exception . Instead, if the object can t be cast, the result is null . You ll want to check to see whether the resulting reference is null, or attempting to use the resulting reference will cause a System.NullReferenceException to be thrown . The following code demonstrates:
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Object o = new Object(); // Creates a new Object object Employee e = o as Employee; // Casts o to an Employee // The cast above fails: no exception is thrown, but e is set to null. e.ToString(); // Accessing e throws a NullReferenceException.
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To make sure you understand everything just presented, take the following quiz . Assume that these two class definitions exist:
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internal class B { } // Base class
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internal class D : B { // Derived class }
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Now examine the lines of C# code in Table 4-3 . For each line, decide whether the line would compile and execute successfully (marked OK below), cause a compile-time error (CTE), or cause a run-time error (RTE) .
4 Type Fundamentals
TABLE 4-3
Type-Safety Quiz
3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3
Statement
Object o1 = new Object(); Object o2 = new B(); Object o3 = new D(); Object o4 = o3; B b1 = new B(); B b2 = new D(); D d1 = new D(); B b3 = new Object(); D d2 = new Object(); B b4 = d1; D d3 = b2; D d4 = (D) d1; D d5 = (D) b2; D d6 = (D) b1; B b5 = (B) o1; B b6 = (D) b2;
Note C# allows a type to define conversion operator methods as discussed in the Conversion
Operator Methods section of 9, Parameters . These methods are invoked only when using a cast expression; they are never invoked when using C#'s as or is operator .
Namespaces and Assemblies
Namespaces allow for the logical grouping of related types, and developers typically use them to make it easier to locate a particular type . For example, the System.Text namespace defines a bunch of types for performing string manipulations, and the System.IO namespace defines a bunch of types for performing I/O operations . Here s some code that constructs a System.IO.FileStream object and a System.Text.StringBuilder object:
public sealed class Program { public static void Main() { System.IO.FileStream fs = new System.IO.FileStream(...); System.Text.StringBuilder sb = new System.Text.StringBuilder(); } }
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