Accessing properties of javascript objects using type dynamic in C# 4

Question

I define a com object as a dynamic type in c# I am able to call methods quite easily. However when I try to access a property on the same object I get an invalid cast exception.

The object in question is an array, passed to managed code from JavaScript, and I wish to get the length property of it as an int.

I know I am missing something odd because I am not getting a 'does not contain a definition' exception and I can access the property easily using reflection/InvokeMember.

Why can I not convert the length property of the dynamic type to an int?

For example

This Fails

   dynamic com = comObject;
   int i = com.length; // RTBE here.

This Works

   Type type = comObject.GetType();
   int i = (int)type.InvokeMember("length", BindingFlags.GetProperty, null, comObject, null);

* Update *

After a lot of testing I have narrowed this oddness to cases of multi-dimensional arrays. The com object in question is a parameter passed from a html document to managed code. For all intents and purposes the object sometimes looks like this in JavaScript.

var x = ["a1", "a2", "a3"];

When an array like this comes to managed code I am able to get the length AOK using the type dynamic. (ie the first example here that fails actually works). However, if it is a multi-dimensional array such as the following structure in JavaScript.

var y = [["b1", "b2", "b3"], "a2", "a3"];

Then I get an error when trying to access its length property dynamically. Note, I can still access the length via reflection in this case. It seems to me that for some reason the length property does not get correctly mapped when a multidimensional array is used as a dynmaic type...

In my case what I have done to solve(!?) this is add a 'length_' property to the array like so before passing it.

var y = [["b1", "b2", "b3"], "a2", "a3"];
y.length_ = y.length;

Now in managed code I can accesses this property as expected without error. Far from ideal but seems to work...

   dynamic com = comObject;
   int i = com.length_; // 3!

Further Update

Ok, so it seems that as well as the length property the objects index gets lost to the dynamic type as well. Again it is accessible via reflection though...

Fails

   dynamic com = comObject; // js array i.e. var x = [1, 2];
   int i = com[0]; // MissingMemberException - Error while invoking [PROPERTYGET, DISPID(0)].
   int i = com["0"]; // MissingMemberException - Error while invoking [PROPERTYGET, DISPID(0)].

Works

   Type type = comObject.GetType();
   int i = (int)type.InvokeMember("0", BindingFlags.GetProperty, null, comObject, null); // 1

Answer 1

In simple terms you can't access the length property of a multi-dimensional array in c# via the type dynamic unless, it seems, you have used the length property in JavaScript first...

The simple test below shows this very clearly. I hope this saves someone else the head scratching I have been having over the last day or so.

[ComVisibleAttribute(true)]
public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        webBrowser1.ObjectForScripting = this;

        StringBuilder html = new StringBuilder();
        html.Append("<script>");
        html.Append("var arr1 = [1, 2, 3, 4];");
        html.Append("var arr2 = [arr1, 2, 3, 4];");
        html.Append("var fn1 = function() { return arr1; };");
        html.Append("var fn2 = function() { return arr2; };");
        html.Append("var fn3 = function() { alert(arr2.length); }");
        html.Append("</script>");
        webBrowser1.DocumentText = html.ToString();

        webBrowser1.DocumentCompleted += (o, e) =>
        {
            dynamic arr1 = webBrowser1.Document.InvokeScript("fn1");
            int i = arr1.length;
            MessageBox.Show(i.ToString()); //4

            // If I call fn3 here then the arr2.length *is* available as int i2 below!
            ////webBrowser1.Document.InvokeScript("fn3"); // 4 

            dynamic arr2 = webBrowser1.Document.InvokeScript("fn2");
            int i2 = arr2.length;
            MessageBox.Show(i2.ToString()); // unless fn3 is called you get...
            /* 
            System.MissingMemberException was unhandled by user code
            Message=Error while invoking length.
            Source=System.Dynamic
            StackTrace:
            at System.Dynamic.ComRuntimeHelpers.CheckThrowException(Int32 hresult, ExcepInfo& excepInfo, UInt32 argErr, String message)
            at CallSite.Target(Closure , CallSite , ComObject )
            at System.Dynamic.UpdateDelegates.UpdateAndExecute1[T0,TRet](CallSite site, T0 arg0)
            at CallSite.Target(Closure , CallSite , Object )
            at System.Dynamic.UpdateDelegates.UpdateAndExecute1[T0,TRet](CallSite site, T0 arg0)
            */             
        };
    }
}

update

It seems (see comments) that this behaviour is fixed if the WebBrowser control uses version 9 of Internet Explorer (...the control uses the version of IE on the machine). I can only presume that the IE9 'Chakra' JavaScript engine is doing something extra/different to the old js engine in this case.