Array of JS objects to VB.Net

Question

Bit of a doozy here. I've got various COM exposed systems and I've implemented Google maps (v3) into my .net software. What I'm trying to do now, is that when a user edits a polygon (defining an area), I send back all path points to .net for storing into our database.

My problem is that .Net knows that the JS array I pass back is X elements in size, but for the life of me I can't figure out how to reference the values while iterating through the array.

Here's the .NET (VB) method I'm using from JS with window.external

Public Sub AreaPointMoved(ByRef obj As Object, ByVal s As String)
    MsgBox(s)    ' contains a string of lat/lngs from JS
    MsgBox(obj.length)

    For i As Integer = 0 To obj.length
        MsgBox(obj(i).lat & ", " & obj(i).lng) ' this doesn't work
        'MsgBox(obj.lat & ", " & obj.lng) ' this doesn't work
    Next
End Sub

And the JS that's sending stuff back upon the set_at event being triggered:

    function DrawAreaOverlay(area, col)
    {
        var coordsString = "";
        var areaCoords = [];
        overlayLayer[overlayCount] = new Array();

        for(var j=0; j<area.Count; j++)
        {
            areaCoords.push(new google.maps.LatLng(area.Item(j).lng, area.Item(j).lat));
        }

        var poly = new google.maps.Polygon({
            paths: areaCoords,
            strokeColor: col,
            strokOpacity: 0.35,
            strokeWeight: 2,
            fillColor: col,
            fillOpacity: 0.25,
            geodesic: false,
            editable: canEdit,
            draggable: canDrag,
            map: map
        });

        overlayLayer[overlayCount].push(poly);

        poly.getPaths().forEach(function(path, index){
            google.maps.event.addListener(path, 'set_at', function(){
                var arrayOfPoints = new Array();
                var g = new Array();

                arrayOfPoints = poly.getPath();                         
                coordsString = "";

                for(var i=0; i<arrayOfPoints.length; i++)
                {                       
                    //simpleArray[i] = poly.getPath().getAt(i).lat() + ", " + poly.getPath().getAt(i).lng();
                    geoObj = new Object();
                    geoObj.lat = poly.getPath().getAt(i).lat();
                    geoObj.lng = poly.getPath().getAt(i).lng();

                    g.push(geoObj);

                    coordsString += poly.getPath().getAt(i).lat() + ", " + poly.getPath().getAt(i).lng() + "|";
                }

                window.external.AreaPointMoved(g, coordsString);
                //alert(path.getLength());
            });
        });
    }

I'm really confused. Getting objects from .net to JS was a doddle. But I can't for the life of me figure out what I'm doing wrong on the reverse :(

Cheers.

What if you put a breakpoint on the MsgBox and inspect whats in there? — EaterOfCode
– EaterOfCode, Commented Jan 28, 2014 at 12:42
Displays: System.__COMObject - But wont let me see any fields or properties of any kind. — NoGall
– NoGall, Commented Jan 28, 2014 at 12:46
Doesn't even compile. "Identifier expected." using: obj[0].lat - I'll get around it with Strings. The old school way! I was just hoping that because it was simple getting an enumerable array across to JS, it'd be easy returning and iterating one from JS. — NoGall
– NoGall, Commented Jan 28, 2014 at 13:37
A similar q/a: stackoverflow.com/a/20943173/1768303. The code is in C#, but the same Reflection API can be used from VB.NET. — noseratio
– noseratio, Commented Jan 29, 2014 at 9:59

Community · Accepted Answer · 2017-05-23 11:49:24Z

According to answer like this, you have two options at your disposal for interacting with non-primitive arguments with calls to your ObjectForScripting. This Connect thread says you should use a dynamic as the argument type.

To my surprise, I learned that VB.NET doesn't have a an equivalent to C#'s dynamic keyword. Apparently Option Strict Off and Option Infer On accomplish the same ends:

Option Strict Off
Option Infer On

Public Sub AreaPointMoved(ByRef obj As Object, ByVal s As String)
    For i As Integer = 0 To obj.length
        'late binding to .lat and .lng should work now
        ' open Debug > Windows > Immediate
        Debug.Print(obj(i).lat) 
        Debug.Print(obj(i).lng)
   Next
 End Sub

This is untested as I can't easily test any of this code. If this doesn't work, the second hail mary is using reflection to invoke the property:

Public Sub AreaPointMoved(ByRef obj As Object, ByVal s As String)
    Dim lat As Double
    Dim lng As Double


    For i As Integer = 0 To obj.length
        lat = obj(i).GetType().InvokeMember("lat", BindingFlags.InvokeMethod, Nothing, obj(i), Nothing)
        lng = obj(i).GetType().InvokeMember("lng", BindingFlags.InvokeMethod, Nothing, obj(i), Nothing)
        ' view in Debug > Windows > Immediate
        Debug.Print(lat) 
        Debug.Print(lng)
   Next
 End Sub

Forgive my VB.NET for any syntax errors. My skills translating C# to VB.NET are rusty at best.

Have you thought about using an array of arrays of doubles? You should not need an object property just to pass coordinates:

Public Sub AreaPointMoved(ByVal coordinates()() As Double)
End Sub

 for(var i=0; i<arrayOfPoints.length; i++)
 {                       
      g.push([poly.getPath().getAt(i).lat(),  poly.getPath().getAt(i).lng()]);
 }
 window.external.AreaPointMoved(g);

Hopefully one of these will help you sort this out. Good luck!

Sidebar: I highly recommend using Chromium Embedded Framework (CEF) or possibly Awesomium over the Windows WebBrowser control (which is a COM interop wrapper around Internet Explorer). Embedding Google Maps into a desktop app is going to be way smoother with a browser based on Chromium over IE. Both CEF and Awesomium have much richer ways to accomplish bi-directional calls between .Net IL and Javascript. You are having a hard time with the window.external and WebBrowser.ObjectForScripting simply because it is a crappy API for anything serious.

Agreed that the IE browser wrapper isn't overly pleasant to use! But it's mainly a prototype at the moment. Thanks for the links, I'll take a look and see what I can do. Most of the grunt work is done within the JS itself, so hopefully it'll just be a case of migrating controls and a different way of loading the filesystem content. Cheers! +1

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