Classes and Indexers
|
Classes and Indexers |
|
Fundamentals of Indexed Properties and Classes |
In the previous lesson, we learned to create and use indexer that were taking parameters of primitive types. Just as we did with primitive types, you can create an indexer that is of a class type. For example, you can create a class so that one of the its fields declared as an array can be accessed with an index directly applied to an instance of the class. |
Before designing an indexer that is class-based, first create the class that will be used as the data type. The class can be simple or complex as you judge it necessary. Here is an example of a simple class: public class Student
{
public string FirstName;
public string LastName;
public int Gender;
}
When creating the class that will host the indexed property, declare an array field for the class. Then, create the this property with the desired accessor(s). Here is an example: 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 Program
{
static int Main()
{
SchoolRegistration pupils = new SchoolRegistration();
for (int 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 Program
{
static int Main()
{
SchoolRegistration pupils = new SchoolRegistration();
Student 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 . . . |
|
|
||
| Home | Copyright © 2007-2013, FunctionX | Next |
|
|
||
Practical
Learning: Introducing Indexers and Classes