How to write a class that (like array) can be indexed with `arr[key]`?

Question

Like we do Session.Add("LoginUserId", 123); and then we can access Session["LoginUserId"], like an Array, how do we implement it?

score 63 · Accepted Answer · edited May 23 '17 at 11:47

63

You need an indexer:

public Thing this[string index]
{
    get
    {
         // get the item for that index.
         return YourGetItemMethod(index)
    }
    set
    {
        // set the item for this index. value will be of type Thing.
        YourAddItemMethod(index, value)
    }
}

This will let you use your class objects like an array:

MyClass cl = new MyClass();
cl["hello"] = anotherObject;
// etc.

There's also a tutorial available if you need more help.

Addendum:

You mention that you wanted this to be available on a static class. That get's a little more complicated, because you can't use a static indexer. If you want to use an indexer, you'd need to access it off of a static Field or some such sorcery as in this answer.

edited May 23 '17 at 11:47

Community

1
1

answered Jul 24 '11 at 14:13

jakebasile

8,084
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why not inherit from Dictionary ? – Mo Valipour Jul 24 '11 at 14:29
I gave the most basic way to do what he wanted. I don't know how he stores the collected objects, so he can use anything he likes from within an Indexer. Inheriting from Dictionary will just make what amounts to an alias for a Dictionary of predetermined types unless he overrides the right methods. – jakebasile Jul 24 '11 at 14:35
@Jake Basile, How do I write Add method? and unlike Indexers, I wouldn't like to create its object and using it like static class. – Riz Jul 24 '11 at 14:36
@eFriend, what do you need to do that you can't do with a normal Dictionary? – Can Gencer Jul 24 '11 at 14:38
Normal dictionary is your solution (it has Add, Remove, etc. and also indexer[string]), if you need specific actions for your class, create a class and inherit it from Dictionary. Sometimes I see strange things in SO, I proposed this answer and I got -3 !!! – Mo Valipour Jul 24 '11 at 14:46
@valipour, I agree with you. Sometimes it's better not to give the original poster of the question exactly what they **think** they want! just like the customers ;) But I believe your answer was downvoted because you just pasted code with no context. – Can Gencer Jul 24 '11 at 15:09

Can Gencer · Answer 2 · 2011-07-24T15:13:54.263

Sounds like all you need is a generic dictionary.

var session = new Dictionary<string, object>();

//set value
session.Add("key", value);

//get value
var value = session["key"] as string;

If you want to make this static, just make it a static member in another class.

public static class SharedStorage
{
   private static Dictionary<string, object> _data = new Dictionary<string,object>();
   public static Dictionary<string, object> Data { get { return _data; } }
}

Then you can access it as such, without having to initialize it:

SharedStorage.Data.Add("someKey", "someValue");
string someValue = (string) SharedStorage.Data["someKey"];

If you want to be more adventurous and are using .NET 4 you can also use an Expando Object, like the ViewBag member available to controllers in ASP.NET MVC 3:

dynamic expando = new ExpandoObject();
expando.UserId = 5;
var userId = (int) expando.UserId;

score 2 · Answer 3 · answered Jul 24 '11 at 14:47

2

You should use indexers See the link: http://msdn.microsoft.com/en-us/library/2549tw02.aspx

answered Jul 24 '11 at 14:47

Mohammed Asaad

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Paul Sasik · Answer 4 · 2011-07-24T14:25:27.843

0

With the way you usually use the Session variable all you really need is a generic Dictionary collection like this one. You don't really need to write a class. But if you need to add extra functionality and/or semantics you could certainly wrap the collection with a class and just include and indexer.

For other collections check out the Collections.Generic namespace.

edited Jul 24 '11 at 14:25

answered Jul 24 '11 at 14:15

Paul Sasik

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I want a simple class like Session. I should be able to a) Use without creating object, like static b) Using Add method to add elements c) accessing like array how can I achieve this? – Riz Jul 24 '11 at 14:41
1

Session in this case *is* an object, though. – jakebasile Jul 24 '11 at 14:56
@eFriend, take a look at my answer to see how you would do this in a static way. – Can Gencer Jul 24 '11 at 15:15

How to write a class that (like array) can be indexed with `arr[key]`?

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