public class Student
{
public string firstName;
public string lastName;
public int Gender;
}
public class SchoolRegistration
{
Student[] std = new Student[5];
public Student this[int i]
{
get { return std[i]; }
}
}
After creating the indexing class, you can use it and access the indexer. For example, you can retrieve its value(s). Here is an example: using System;
public class Student
{
public string firstName;
public string lastName;
public int Gender;
}
public class SchoolRegistration
{
Student[] std = new Student[5];
public Student this[int i]
{
get { return std[i]; }
}
public SchoolRegistration()
{
std[0] = new Student();
std[0].firstName = "Alfredo";
std[0].lastName = "Olmos";
std[0].Gender = 2;
std[1] = new Student();
std[1].firstName = "Patricia";
std[1].lastName = "Katts";
std[1].Gender = 1;
std[2] = new Student();
std[2].firstName = "Josiane";
std[2].lastName = "Euler";
std[2].Gender = 1;
std[3] = new Student();
std[3].firstName = "Joan";
std[3].lastName = "Jones";
std[3].Gender = 3;
std[4] = new Student();
std[4].firstName = "George";
std[4].lastName = "Paulson";
std[4].Gender = 2;
}
}
public class Exercise
{
static int Main(string[] args)
{
var pupils = new SchoolRegistration();
for (var i = 0; i < 5; i++)
{
Student pupil = pupils[i];
Console.WriteLine("Student Information");
Console.WriteLine("---------------------");
Console.WriteLine("First Name: {0}", pupil.firstName);
Console.WriteLine("Last Name: {0}", pupil.lastName);
Console.WriteLine("Gender: {0}\n",
(pupil.Gender == 1 ? "Female" :
(pupil.Gender == 2 ? "Male" : "Unknown")));
}
return 0;
}
}
This would produce: Student Information --------------------- First Name: Alfredo Last Name: Olmos Gender: Male Student Information --------------------- First Name: Patricia Last Name: Katts Gender: Female Student Information --------------------- First Name: Josiane Last Name: Euler Gender: Female Student Information --------------------- First Name: Joan Last Name: Jones Gender: Unknown Student Information --------------------- First Name: George Last Name: Paulson Gender: Male Press any key to continue . . .
The above implementation of the SchoolRegistration class easily allowed us to locate an element of the array by specifying its integer-based index. As done for primitive types, an indexer can take a parameter other than an integer. In some cases, you may use your class or a class created by someone else and need to access an element of the array without information other than its index. Consider the following program: public enum Classification
{
Female,
Male,
Unknown
}
public class Student
{
public long StudentID;
public string firstName;
public string lastName;
public Classification Gender;
public override string ToString()
{
string str = "Student ID: " + StudentID +
"\nFirst Name: " + firstName +
"\nLast Name: " + lastName +
"\nGender: " + Gender;
return str;
}
}
public class SchoolRegistration
{
Student[] std = new Student[50];
public Student this[...]
{
}
}
Previously, we saw that you could create an indexer that takes a type than an integer. For example, we saw that a string could be used as an index. By now, we know that a basic indexed property produces (or all the indexed properties we have studied so far produce) only one value. If you have a class that has only one field, this would be enough. In reality, most of the time, a class has many fields. In such a case, when you create an indexer , you need to be able to refer to one exact element of the array. To make this possible, you must define a way to point to the particular element you want. One way you can do this is to use one field of the class as a reference. This is better if that field holds unique values among the other elements of the array. For our Student class, we could use the StudentID field (because we will make sure that each student has a unique ID). You can start the property as follows: public class SchoolRegistration
{
Student[] std = new Student[5];
public Student this[long id]
{
}
}
When a user uses this property, he or she must provide a value that uniquely identifies an element of the array. You in turn, when you get this value, you can search for it in the array. If you find it and the array has a get accessor, you can then return the desired but appropriate value. Here is how this can be done: public class SchoolRegistration
{
Student[] students = new Student[50];
public Student this[long id]
{
get
{
for (int i = 0; i < students.Length; i++)
{
if (students[i].StudentID == id)
return students[i];
}
// Unknown student or the number was not found
return null;
}
}
}
After creating the indexer, you can use it. Once again, you must follow the rules of a method that takes an argument and returns a value other than void. In this case, the indexer must take a string and it must return a Student object. Here is an example: using System;
public enum Classification
{
Female,
Male,
Unknown
}
public class Student
{
public long StudentID;
public string firstName;
public string lastName;
public Classification Gender;
public override string ToString()
{
string str = "Student ID: " + StudentID +
"\nFirst Name: " + firstName +
"\nLast Name: " + lastName +
"\nGender: " + Gender;
return str;
}
}
public class SchoolRegistration
{
Student[] students = new Student[50];
public Student this[long id]
{
get
{
for (int i = 0; i < students.Length; i++)
{
if (students[i].StudentID == id)
return students[i];
}
// Unknown student or the number was not found
return null;
}
}
public SchoolRegistration()
{
students[0] = new Student();
students[0].StudentID = 917294;
students[0].firstName = "Helene";
students[0].lastName = "Mukoko";
students[0].Gender = Classification.Female;
students[1] = new Student();
students[1].StudentID = 283764;
students[1].firstName = "Patrice";
students[1].lastName = "Katts";
students[1].Gender = Classification.Unknown;
students[2] = new Student();
students[2].StudentID = 192046;
students[2].firstName = "Armand";
students[2].lastName = "Essono";
students[2].Gender = Classification.Male;
students[3] = new Student();
students[3].StudentID = 618268;
students[3].firstName = "Bertrand";
students[3].lastName = "Yamaguchi";
students[3].Gender = Classification.Male;
students[4] = new Student();
students[4].StudentID = 820648;
students[4].firstName = "Hortense";
students[4].lastName = "McNeal";
students[4].Gender = Classification.Female;
}
}
public class Exercise
{
static int Main(string[] args)
{
var pupils = new SchoolRegistration();
var pupil = pupils[820648];
Console.WriteLine("Student Information");
Console.WriteLine("---------------------");
Console.WriteLine("First Name: {0}", pupil.firstName);
Console.WriteLine("Last Name: {0}", pupil.lastName);
Console.WriteLine("Gender: {0}\n", pupil.Gender);
pupil = pupils[192046];
Console.WriteLine("Student Information");
Console.WriteLine("---------------------");
Console.WriteLine("First Name: {0}", pupil.firstName);
Console.WriteLine("Last Name: {0}", pupil.lastName);
Console.WriteLine("Gender: {0}\n", pupil.Gender);
return 0;
}
}
This would produce: Student Information --------------------- First Name: Hortense Last Name: McNeal Gender: Female Student Information --------------------- First Name: Armand Last Name: Essono Gender: Male Press any key to continue . . .
As opposed to returning a class, an indexer can use a class as its index. When creating such a property, the primary action you must take is to include a class and its name as a parameter to the this property. You can start such a class as follows: using System;
public enum Classification
{
Female,
Male,
Unknown
}
public class Student
{
public long StudentID;
public string firstName;
public string lastName;
public Classification Gender;
}
public class SchoolRegistration
{
public string this[Student std]
{
}
}
When implementing the class, you should proceed the same way we have done so far following the rules of a method that takes an argument and returns a value other than void. Here is an example: public class SchoolRegistration
{
Student[] students = new Student[50];
public string this[Student std]
{
get
{
for (int i = 0; i < students.Length; i++)
{
if (std.StudentID == students[i].StudentID)
return "Student ID: " + students[i].StudentID +
"\nFirst Name: " + students[i].firstName +
"\nLast Name: " + students[i].lastName +
"\nGender: " + students[i].Gender;
}
// Unknown student or the number was not found
return "";
}
}
}
After creating the property, you can use it. To do this, you must pass an object that is the type of the index. You can then use the returned value as you see fit. Here is an example: using System;
public enum Classification
{
Female,
Male,
Unknown
}
public class Student
{
public long StudentID;
public string firstName;
public string lastName;
public Classification Gender;
}
public class SchoolRegistration
{
Student[] students = new Student[50];
public string this[Student std]
{
get
{
for (int i = 0; i < students.Length; i++)
{
if (std.StudentID == students[i].StudentID)
return "Student ID: " + students[i].StudentID +
"\nFirst Name: " + students[i].firstName +
"\nLast Name: " + students[i].lastName +
"\nGender: " + students[i].Gender;
}
// Unknown student or the number was not found
return "";
}
}
public SchoolRegistration()
{
students[0] = new Student();
students[0].StudentID = 917294;
students[0].firstName = "Helene";
students[0].lastName = "Mukoko";
students[0].Gender = Classification.Female;
students[1] = new Student();
students[1].StudentID = 283764;
students[1].firstName = "Patrice";
students[1].lastName = "Katts";
students[1].Gender = Classification.Unknown;
students[2] = new Student();
students[2].StudentID = 192046;
students[2].firstName = "Armand";
students[2].lastName = "Essono";
students[2].Gender = Classification.Male;
students[3] = new Student();
students[3].StudentID = 618268;
students[3].firstName = "Bertrand";
students[3].lastName = "Yamaguchi";
students[3].Gender = Classification.Male;
students[4] = new Student();
students[4].StudentID = 820648;
students[4].firstName = "Hortense";
students[4].lastName = "McNeal";
students[4].Gender = Classification.Female;
students[5] = new Student();
students[5].StudentID = 917394;
students[5].firstName = "Alfredo";
students[5].lastName = "Olmos";
students[5].Gender = Classification.Unknown;
students[6] = new Student();
students[6].StudentID = 163864;
students[6].firstName = "Josiane";
students[6].lastName = "Euler";
students[6].Gender = Classification.Female;
students[7] = new Student();
students[7].StudentID = 826384;
students[7].firstName = "Joan";
students[7].lastName = "Jones";
students[7].Gender = Classification.Female ;
}
}
public class Exercise
{
static int Main(string[] args)
{
var pupils = new SchoolRegistration();
var pupil = new Student();
pupil.StudentID = 820648;
var strStudent = pupils[pupil];
Console.WriteLine("=====================");
Console.WriteLine("Student Information");
Console.WriteLine("---------------------");
Console.WriteLine(strStudent);
//pupil = new Student();
pupil.StudentID = 192046;
strStudent = pupils[pupil];
Console.WriteLine("=====================");
Console.WriteLine("Student Information");
Console.WriteLine("---------------------");
Console.WriteLine(strStudent);
Console.WriteLine("=====================\n");
return 0;
}
}
This would produce: ===================== Student Information --------------------- Student ID: 820648 First Name: Hortense Last Name: McNeal Gender: Female ===================== Student Information --------------------- Student ID: 192046 First Name: Armand Last Name: Essono Gender: Male ===================== Press any key to continue . . . You can also directly pass an instance of the class in the square brackets of the object that holds the indexed property, as long as you specify the object. Here is an example: using System;
public enum Classification
{
Female,
Male,
Unknown
}
public class Student
{
public long StudentID;
public string firstName;
public string lastName;
public Classification Gender;
public Student()
{
}
public Student(long id)
{
this.StudentID = id;
}
}
public class SchoolRegistration
{
Student[] students = new Student[50];
public string this[Student std]
{
. . . No Change
}
public SchoolRegistration()
{
. . . No Change
}
}
public class Exercise
{
static int Main(string[] args)
{
var pupils = new SchoolRegistration();
var strStudent = pupils[new Student(618268)];
Console.WriteLine("=====================");
Console.WriteLine("Student Information");
Console.WriteLine("---------------------");
Console.WriteLine(strStudent);
Console.WriteLine("=====================\n");
return 0;
}
}
This would produce: ===================== Student Information --------------------- Student ID: 618268 First Name: Bertrand Last Name: Yamaguchi Gender: Male ===================== Press any key to continue . . .
As mentioned for indexers that return primitive types, you can overload an indexed property that produces a class. You do this following the same rules applied to method overloading and arrays:
As done for a primitive type, you can allow the clients of your indexer to assign values to the array's elements. Once again, when defining the property, you should include a set accessor to it. In the set accessor, you should assign the value keyword to an element of the array. Here is an example: public class SchoolRegistration
{
Student[] std = new Student[5];
public Student this[int i]
{
get { return std[i]; }
set { std[i] = value; }
}
}
After doing this, you can create an element of the array by applying the square brackets to the instance of the class and assigning the desired value to it. The problem with the class is that, since it may have many fields (or properties), to completely define each element, you must provide a value to the member variables of the class itself. Here is an example: using System;
public enum Classification
{
Female,
Male,
Unknown
}
public class Student
{
public long StudentID;
public string firstName;
public string lastName;
public Classification Gender;
public override string ToString()
{
string str = "Student ID: " + StudentID +
"\nFirst Name: " + firstName +
"\nLast Name: " + lastName +
"\nGender: " + Gender;
return str;
}
}
public class SchoolRegistration
{
Student[] std = new Student[5];
public Student this[int i]
{
get { return std[i]; }
set { std[i] = value; }
}
}
public class Exercise
{
static int Main(string[] args)
{
var registration = new SchoolRegistration();
var stud = new Student();
stud.StudentID = 604057;
stud.firstName = "Gertrude";
stud.lastName = "Monayong";
stud.Gender = Classification.Female;
registration[2] = stud;
Console.WriteLine("Student Information");
Console.WriteLine("---------------------");
Console.WriteLine("First Name: {0}", registration[2].firstName);
Console.WriteLine("Last Name: {0}", registration[2].lastName);
Console.WriteLine("Gender: {0}\n",registration[2].Gender);
return 0;
}
}
This would produce: Student Information --------------------- First Name: Gertrude Last Name: Monayong Gender: Female Press any key to continue . . .
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Application:
Introducing Indexers and Classes