Indexers.

Transcription

1 Indexers Indexers are special type of class members that provide the mechanism by which an object can be indexed like an array. Main use of indexers is to support the creation of specialized array that are subject to one or more constraints. Indexers, however, can be used for any purpose for which an array-like syntax is beneficial. Indexers, like array can have one or more dimensions. 253

2 Creating One-dimensional Indexers element-type this [ int index ] //the get accessor get // return the value specified by the index //the set accessor set // set the vale specified by the index 254

3 Here, element-type is the element type of the indexer. Each element accessed by the indexer will be of type element-type. This type corresponds to the element type of an array. The parameter index receives the index of element being accessed. Inside the body of indexer, two accessors are defined, get and set. An accessor is similar to a method except that it does not declare a return type or parameters. Accessors are automatically called when the indexers are used, and both accessors receives index as a parameter. If the indexer is on the left side of an assignment operator, set accessor is called and the element specified by the index must be set. Otherwise, the get accessor is called, and the value associated with index must be returned. The set method also implicitly receives a parameter called value which contains the value being assigned to the specific index. 255

4 An indexer is declared d like a property except that t the name of the member is this followed by a parameter list written between the delimiters [ and ]. The parameters are available in the accessor(s) of the indexer. Defining a class to hold the list of runners in an athletics race. The runners are held in lane order, and an indexer is exposed which allows the list to be both read from and written to. The indexer deals gracefully with cases in which the lane number passed to it is either too high h or too low. 256

5 Class RaceDetails private string[] lanes; public RaceDetails() this.lanes = new string[8]; public string this [int i] get return (i>=0 && i<8)?this.lanes[i]: error ; set if( i>=0 && i<8 ) this.lanes[i] = value; Class UseRaceDetails static void Main() RaceDetails rd = new RaceDetails(); rd[0] = fred ; Console.WriteLine( Lane Lane One : +rd[0]); 257

6 Indexers aren t differentiated by name, and a class cannot declare two indexers with the same signature. But this does not means that a class is limited to just one indexer. Different indexers can have different types and numbers of indexing elements. This is equivalent to method parameters, except that each indexer must have at least one indexing element, and the ref & out modifiers cannot be used. 258

7 using System; class indexerclass private int[] myarray; public indexerclass() this.myarray = new int [100]; public int this [int index] //Indexer Decln. get // check the index limits if ( index <0 index >= 100) return 0; else return this.myarray [ index ]; set if (! (index < 0 index >= 100)) this.myarray [ index ] = value; public class MainClass public static void Main() indexerclass b = new indexerclass(); /*call the indexer to initialize the elements number 3 and 5. */ b[3] = 256; b[5] = 1024; for ( int i = 0;i<10;i++) ; i++ Console.WriteLine( Element # 0 = 1,i,b[i]); 259

8 Output: Element # 0 = 0 Element # 1 = 0 Element # 2 = 0 Element # 3 = 256 Element # 4 = 0 Element # 5 = 1024 Element # 6 = 0 Element # 7 = 0 Element # 8 = 0 Element # 9 = 0 Element # 10 = 0 260

9 Property Indexer Identified d by its name. Identified d by its signature. Accessed through a simple Access through an element name or a member access. access. Can be a static or instance member. A get accessor of a property has no parameters. A set accessor of a property contains the implicit value parameter. Must be an instance member. A get accessor of an indexer has the same formal parameter list as the indexer. A set accessor of an indexer has the same formal parameter list as the indexer, in addition to the value. 261

10 class Employee private string m_firstname; private string m_middlename; private string m_lastname; private string m_ssn; public Employee(string fn, string mn, string ln, string ssn) m_firstname = fn; m_middlename= mn; m_lastname = ln; m_ssn = ssn; public string FirstName get return m_firstname; set m_firstname = value; public string MiddleName get return m_middlename; set m_middlename = value; public string LastName get return m_lastname; set m_lastname = value; public string SSN get return m_ssn; set m_ssn = value; 1. Employee.cs 2. Employees.cs 3 I 3. IndexersSample.cs 262

11 class Employees private ArrayList m_employees; private int m_maxemployees; //constructor public Employees(int MaxEmployees) m_maxemployees = MaxEmployees; m_employees = new ArrayList(MaxEmployees); //Indexer - 1; by Array Index public Employee this[int index] get //check for out of bounds condition if (index < 0 index > m_employees.count - 1) return null; return (Employee)m_Employees[index]; set //check for out of bounds condition if (index < 0 index > m_maxemployees - 1) return; m_employees.insert(index, value); // CLASS EMPLOYEES: CODE CONTINUED IN NEXT PAGE 1. Employee.cs 2. Employees.cs (continued in next page) 3. IndexersSample.cs 263

12 // CLASS EMPLOYEES: CODE CONTINUED FROM PREVIOUS PAGE //Indexer - 2; by SSN public Employee this[string SSN] get Employee empreturned = null; foreach (Employee employee in m_employees) if (employee.ssn == SSN) empreturned = employee; break; return empreturned; //Property to return the total number of employees. public int Length get return m_employees.count; 1. Employee.cs 2. Employees.cs (complete) 3 I 3. IndexersSample.cs 264

13 class IndexersSample static void Main(string[] args) Employees employees = new Employees(4); //adding employees now employees[0]=new Employee("Andrew","Arthur","Tucker","000"); employees[1]=new Employee("Sally","Jones","Mathew","111"); employees[2]=new Employee("John","Arthur","Tucker","222"); //displaying the list for (int i = 0; i < employees.length; i++) string Name = employees[i].firstname + " " + employees[i].middlename + " " + employees[i].lastname; string SSn = employees[i].ssn; Console.WriteLine("Name:0,SSN:1", Name, SSn); //search using Indexer by SSN Employee emp = employees["222"]; if (emp!= null) string name = emp.firstname + "" + emp.middlename + "" + emp.lastname; string ssn = emp.ssn; Console.WriteLine("Found by SSN: Name:0, SSN:1", name, ssn); else Console.WriteLine("Not Found by SSN: " + emp.ssn); Console.ReadKey(); 1. Employee.cs 2. Employees.cs 3. IndexersSample.cs 265