Why does web service return data as msg.d

Question

I found that my webmethod is returning data as

{ "d":
 [
   {"id":"1","itemtxt":"Masters"},
   {"id":"2","itemtxt":"Transactions"},
   {"id":"3","itemtxt":"Misch. Reports"}
 ]
}

If you notice, the array is named as "d". Why is that ? is there any rule for it?

For your information I am returning a list of objects (List<webitem>)

public class webitem
{
    public webitem(string kid, string kval)
    {
        this.id = kid;
        this.itemtxt = kval;
    }

    public string id { get; set;  }
    public string itemtxt { get; set; }
}

What does this "d" mean ? will it allways be same for whatever data i send from my webmethod? or it is going to change depending on my data type/class type?

EDIT

Here is the webmethod

[WebMethod]
public  List<webitem> GetSubModules(string key)
{
    string s = " order by smid";
    if (key != null)
        s = " where moduleid=" + key + s;
    return Utility.GetDDLVal("Select smid id,smname itemtxt from m_submodules "+s, null, null);
}

Answer 1

It's a bit of a security feature as part of ASP.NET. It will always be there as long as you don't change serializers. David Ward's blog (now defunct) said what it is:

This is the case with all ASMX services JSON serialized through the ASP.NET AJAX Extensions in ASP.NET 3.5. Even if you’re only returning a scalar return value, such as a string, int, or boolean, the result will always be enclosed within the “d”.

And the why:

{"d": 1 }

Is not a valid JavaScript statement, where as this:

[1]

Is.

So the wrapping of the "d" parameter prevents direct execution of the string as script. No Object or Array constructor worries.