#366 – Defining an Indexer with More than One Parameter

An indexer will typically have a single parameter, often based on an integer type.

        public LogMessage this[int i]
        {
            get { return messages[i]; }
            set { messages[i] = value; }
        }

You can also define an indexer that takes more than one parameter.  In the example below, we have an internal data structure that stores a list of messages for each day of the week.  We can retrieve a specific message by passing in a parameter representing the day, as well as a 0-based index into the list of messages for that day.

            // Get 1st Saturday message
            LogMessage msg = log[Days.Sat, 0];

Here’s what the definition of the indexer looks like.

        private Dictionary<Days, List<LogMessage>> dailyMessages = new Dictionary<Days, List<LogMessage>>();

        public LogMessage this[Days day, int i]
        {
            get { return dailyMessages[day][i]; }
        }

#365 – Overloading an Indexer

You can define several versions of an indexer in a class, each indexed using a different type, overloading the indexer.

In the example below, we can index using the Days enumerated type or a 0-based integer representing the day.

    public class Logger
    {
        private Dictionary<Days, LogMessage> dailyMessages = new Dictionary<Days, LogMessage>();

        public LogMessage this[Days day]
        {
            get { return dailyMessages[day]; }
            set { dailyMessages[day] = value; }
        }

        public LogMessage this[int i]
        {
            get { return dailyMessages[(Days)i]; }
            set { dailyMessages[(Days)i] = value; }
        }
    }

Using this class, we could then index using either the Days or the integer type.

            Logger log = new Logger();

            log[Days.Mon] = new LogMessage("Monday was a good day");
            log[2] = new LogMessage("Tuesday not bad either");

In this example, we indexed into the same internal data object.  But we could have also indexed into different objects, depending on the indexer’s type.