Exposing the indexer / default property via COM Interop

Question

I am attempting to write a component in C# to be consumed by classic ASP that allows me to access the indexer of the component (aka default property).

For example:
C# component:

public class MyCollection {
    public string this[string key] {
        get { /* return the value associated with key */ }
    }

    public void Add(string key, string value) {
        /* add a new element */
    }
}

ASP consumer:

Dim collection
Set collection = Server.CreateObject("MyCollection ")
Call collection.Add("key", "value")
Response.Write(collection("key")) ' should print "value"

Is there an attribute I need to set, do I need to implement an interface or do I need to do something else? Or this not possible via COM Interop?

The purpose is that I am attempting to create test doubles for some of the built-in ASP objects such as Request, which make use of collections using these default properties (such as Request.QueryString("key")). Alternative suggestions are welcome.

Update: I asked a follow-up question: Why is the indexer on my .NET component not always accessible from VBScript?

Answer 1

Try setting the DispId attribute of the property to be 0, as described here in the MSDN documentation.

Answer 2

Here is a better solution that uses an indexer rather than an Item method:

public class MyCollection {
    private NameValueCollection _collection;

    [DispId(0)]
    public string this[string name] {
        get { return _collection[name]; }
        set { _collection[name] = value; }
    }
}

It can be used from ASP like:

Dim collection
Set collection = Server.CreateObject("MyCollection")
collection("key") = "value"
Response.Write(collection("key")) ' should print "value"

Note: I could not get this to work earlier because I had overloaded the indexer, this[string name], with this[int index].

Answer 3

Thanks to Rob Walker's tip, I got it working by adding the following method and attribute to MyCollection:

[DispId(0)]
public string Item(string key) {
    return this[key];
}

Edit: See this better solution which uses an indexer.