Custom use of indexers []

Question

I want to create an object that will work in a similar way to ASP.Net Session.

Say I call this object mySession, I want to make it so when you do

mySession["Username"] = "Gav"

It will either add it to a database table if it doesnt exist or update it if it does. I can write a method to do this but have no idea how to get it to fire when used with the indexer syntax ([]). I've never built an object that does anything like this with indexers.

Before anyone says anything I know the ASP.Net session can save to database but in this case I need a slightly simpler customized solution.

Any pointers or examples of using indexers in this way would be great.

Thanks

Answer 1

It's actually pretty much the same as writing a typical property:

public class MySessionThing
{
    public object this[string key]
    {
        //called when we ask for something = mySession["value"]
        get
        {
            return MyGetData(key);
        }
        //called when we assign mySession["value"] = something
        set
        {
            MySetData(key, value);
        }
    }

    private object MyGetData(string key)
    {
        //lookup and return object
    }

    private void MySetData(string key, object value)
    {
        //store the key/object pair to your repository
    }
}

The only difference is we use the keyword "this" instead of giving it a proper name:

public          object            MyProperty
^access         ^(return) type    ^name
 modifier

public          object            this
^ditto          ^ditto            ^ "name"

Answer 2

Indexers in C# are properties with the name this. Here's an example...

public class Session {
   //...
   public string this[string key]
   {
      get { /* get it from the database */ }
      set { /* store it in the database */ }
   }
}

Answer 3

From the MSDN documentation:

class SampleCollection<T>
{
    // Declare an array to store the data elements.
    private T[] arr = new T[100];

    // Define the indexer, which will allow client code
    // to use [] notation on the class instance itself.
    // (See line 2 of code in Main below.)        
    public T this[int i]
    {
        get
        {
            // This indexer is very simple, and just returns or sets
            // the corresponding element from the internal array.
            return arr[i];
        }
        set
        {
            arr[i] = value;
        }
    }
}

// This class shows how client code uses the indexer.
class Program
{
    static void Main(string[] args)
    {
        // Declare an instance of the SampleCollection type.
        SampleCollection<string> stringCollection = new SampleCollection<string>();

        // Use [] notation on the type.
        stringCollection[0] = "Hello, World";
        System.Console.WriteLine(stringCollection[0]);
    }
}