An indexer allows an object to be indexed such as an array.
When you define an indexer for a class, this class behaves similar to a virtual array.
You can then access the instance of this class using the array access operator ([ ]).
In order to understand the scenario lets create a Person class.
Give some attributes like Id and name to it.
class Person
{
public int EmpId { get; set; }
public string EmpName { get; set; }
}
Now let's create a class Company.
We are already aware of the list class, lets create a Person type list.
After that set some initial values of Id and name using constructor.
In this way we have a “Person” as a data type in the list that we are going to create.
class Company
{
private List listPersons = new List();
public Company()
{
listPersons.Add(new Person { EmpId = 1, EmpName = "Awias" }) ;
listPersons.Add(new Person { EmpId = 2, EmpName = "Ali" });
listPersons.Add(new Person { EmpId = 3, EmpName = "Ahmad" });
}
}
Create Indexer
Now we have almost all the ingredients to create an indexer that can be useful in our example.
Indexers have a return type, they have parameters and get/set accessors.
Now I am going to create a string type indexer with parameter “EmpId”.
public string this[int empId]
After that we have to define get and set accessors.
In get accessor traverse the listPersons and find the id that matches with the parameter passed.
After that return the name against that Id.
get
{
foreach(var per in listPersons)
{
if(per.EmpId == empId)
{
return per.EmpName;
}
}
return null;
}
In set accessor we can find the object which has the id same as the passed parameter and change its name.
set
{
foreach (var per in listPersons)
{
if (per.EmpId == empId)
{
per.EmpName = value;
}
}
}
Overloaded Indexer
We can have multiple indexers in a class, but they must have differrnt parameters.