Question

I’m going through another developer’s code, which is shown below:

[XmlElement("AdminRecipient")] public AdminRecipient[] AdminRecipientCollection = new AdminRecipient[0];

        public AdminRecipient this[ string type ]
        {
            get
            {
                AdminRecipient result = null;
                foreach( AdminRecipient emailRecipient in AdminRecipientCollection )
                {
                    if( emailRecipient.Type == type )
                    {
                        result = emailRecipient;
                        break;
                    }
                }
                return( result );
            }           

Can someone explain what’s going to happen in this line?

public AdminRecipient[] AdminRecipientCollection = new AdminRecipient[0];

The XML file that contains all of the email recipients has about 5 email addresses. But by using [0], will the foreach loop return each of those email addresses?

I have a basic understanding of indexers, but I don’t this. What does it do?:

public AdminRecipient this[ string type ]

At the end of the day, the problem here is that the application doesn’t send out an email when all 5 recipients are in the xml file. If I replace the 5 addresses with just 1 email addresses, then I’m able to get the email (which leads me to believe that there’s a logic issue somewhere here).

Answer 1

An indexer allows you to use a type with the same syntax as array access. One of the simplest examples would be List<T>:

List<string> x = new List<string>();
x.Add("Item 0");
x.Add("Item 1");

string y = x[0]; // "Item 0"

That use of x[0] is calling the indexer for List<T>. Now for List<T> the index is an integer, as it is for an array, but it doesn’t have to be. For example, with Dictionary<TKey, TValue> it’s the key type:

Dictionary<string, string> dictionary = new Dictionary<string, string>();
dictionary.Add("foo", "bar");

string fetched = dictionary["foo"]; // fetched = "bar"

You can have a “setter” on an indexer too:

dictionary["a"] = "b";

Your indexer is just returning the first AdminRecipient in the array with a matching type – or null if no match can be found.

(It’s unfortunate that the code you’ve shown is also using a public field, by the way. It would be better as a property – and probably not an array, either. But that’s a separate discussion.)

EDIT: Regarding the first line you highlighted:

public AdminRecipient[] AdminRecipientCollection = new AdminRecipient[0];

That will create an array with no elements, and assign a reference to the AdminRecipientCollection field. With no further changes, the foreach loop would not have anything to iterate over, and the indexer will always return null.

However, presumably something else – such as XML serialization – is assigning a different value to that field – populating it with more useful data.