When should you use C# indexers?

Question

I\'d like to use indexers more, but I\'m not sure when to use them. All I\'ve found online are examples that use classes like MyClass and IndexerClass

Answer 1

You typically use an indexer if the class represents a list, collection or array of objects. In your case, you could provide an indexer to provide index-based access to a teacher's students.

Answer 2

I remember there was this time when I had a long inside a class, and some digits of that particular long meant something (for example if the third digit was a 3 it meant that the chart had a specific type of encoding, horrible system I know but I didn't invent it)

So I did something like this to return the xth digit of the number:

protected int this[int index]
{
    get
    {
        int tempN = Number;
        if (index > 0)
        {
            tempN /= index * 10;
        }
        return tempN % 10;
    }
}

It was protected because a different method used it, so it was kind of a helper. Of course a simple GetDigit(int a) would've been the same thing (and more self-explanatory) but at the time I thought it was one of the few times I thought using an indexer would make sense. Haven't used them since =(.

Answer 3

Heres a video i have created http://www.youtube.com/watch?v=HdtEQqu0yOY and below is a detailed explanation about the same.

Indexers helps to access contained collection with in a class using a simplified interface. It’s a syntactic sugar.

For instance lets say you have a customer class with addresses collection inside it. Now let’s say we would like to like fetch the addresses collection by “Pincode” and “PhoneNumber”. So the logical step would be that you would go and create two overloaded functions one which fetches by using “PhoneNumber” and the other by “PinCode”. You can see in the below code we have two functions defined.

Customer Customers = new Customer();
Customers.getAddress(1001);
Customers.getAddress("9090");

If you use indexer you can simplify the above code with something as shown in the below code.

Customer Customers = new Customer();
Address o = Customers[10001];
o = Customers["4320948"];

Cheers.

Answer 4

An indexer use in your situation would be a TeachersClass class, which would encapsulate the students (collection) and the current teacher. Although you could do the same thing by exposing the list of students, but it does show you an example.

Here is a code example:

public class TeachersClass
    {
        private List<Student> _students;

        public TeachersClass(Teacher currentTeacher, List<Student> students)
        {
            CurrentTeacher = currentTeacher;
            _students = students;
        }

        public Teacher CurrentTeacher { get; set; }

        public Student this[int studentID]
        {
            get
            {
                return (from s in _students
                        where s.Id = studentID
                        select s).First();
            }
        }   
    }

Answer 5

An indexer is a means to select an element from an aggregate such as an array or collection. While I agree in part with Ian Davis, I think indexers represent something more than public API polish.

Indexers are the primary means of accessing arrays and most of the major classes representing collections in the .NET BCL implemented indexers, presumably to provide a common expernce when dealing with types that aggregate other types.

Because indexers are a standard part of the interface to many of the BCLs collection types, and because these types are heavily used, as developers learn .NET as a platform, it is reasonable to suggest that an expectation is created that collections can be accessed using some type of indexer.

If your type's interface matches the expectations that developers have already, then that type becomes easier to use because the developer doesn't have to think. This is true whether the developers in question are internal to your organization or out there in the wild.

Of course there are situations where having an indexer just doesn't make sense, and if thats the case then don't implement an indexer.

Answer 6

IMHO, Indexers are probably best if you're trying to spruce up a packaged API - it's not worth the effort for your run of the mill business object.

And if you're making some specialized collection, I'd categorize that as sprucing up your packaged API - even if you keep it all in one module.