How do I return an array/list/collection of objects from C# to VB6

Question

I am creating a COM Visible C# object to proxy calls to a webservice for VB6 application. I have a method that returns an array of objects.

public DocActionReport[] DocActionReportByDateRange(System.DateTime reportStartDate, System.DateTime reportEndDate)
{
    object[] results = this.Invoke("DocActionReportByDateRange", new object[] {
                reportStartDate,
                reportEndDate});
    return ((DocActionReport[])(results[0]));
}

When I call this method via VB6, like so:

Dim proxy As New QueueMovementServiceClient.ReadQueueInfo
Dim report() As QueueMovementServiceClient.DocActionReport

report = proxy.DocActionReportByDateRange(startDate, reportEndDate)

It successfully executes (I can see that via logging on the web service) but no data is returned to the object in VB6 (LBound(report) == 0, UBound(report) == -1).

I have tried a couple of different approaches (changing the method to a void method and passing the collection in as a ref parameter) but no joy so far.

What do I need to do to return an array of objects (list, collection, whatever) to a VB6 consumer?

Answer 1

Here's a trick for you:

1. Create the exact same interface with a VB6 Com Object
2. Import that dll into .net
3. User reflector to look at the generated interop interface, this will hopefull allow you to see what types you need to return, then again you might just get object which won't help at all.

In VB6 if my memory goes back far enough, they used something that still leave me with a nervous twitch called SAFEARRAY's.

A SAFEARRAY is probably what needs returning here, have a look at this article, I hope it helps you (its the same issue) ...

How to pass a SAFEARRAY from C# to COM

After reading about SAFEARRAY's my gut feeling is you will decide to return a string and have toJSON and fromJSON parsers on each side of the call ;)