Question

I’d like to create a COM object in C#, and use it via IDispatch from JScript. That part is pretty simple.

I also want to implement simple callbacks on the COM object, similar to the event exposed by the XmlHttpRequest object that is usable in a browser. That model allows Javascript to attach event handlers like this:

var xmlhttp = new ActiveXObject("MSXML.XMLHTTP"); 
xmlhttp.onReadyStateChange = function() {
  ...
};

I want my client-side JScript code to look like this:

var myObject = new ActiveXObject("MyObject.ProgId");
myObject.onMyCustomEvent = function(..args here..) { 
   ...
};

What does the C# code look like? I’d like the general case – I’d like to be able to pass arguments back to the Javascript fn.

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I’ve seen How can I make an ActiveX control written with C# raise events in JavaScript when clicked? , but the answers there look really complicated to implement, and complicated to use.

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From this article, it seems that XMLHttpRequest events are not COM events. The onreadystatechange is a property of type IDispatch. When script clients set that property to a function, JScript marshals it as an IDispatch object.

The only problem that remains is then to invoke the IDispatch from C#.

Answer 1

Since it’s COM, start by defining an interface. Let’s keep it simple.

[Guid("a5ee0756-0cbb-4cf1-9a9c-509407d5eed6")]
[InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
public interface IGreet
{
    [DispId(1)]
    string Hello(string name);

    [DispId(2)]
    Object onHello { get; set; }
}

Then, the implementation:

[ProgId("Cheeso.Greet")]
[ComVisible(true)]
[Guid("bebcfaff-d2f4-4447-ac9f-91bf63b770d8")]
[ClassInterface(ClassInterfaceType.None)]
public partial class Greet : IGreet
{
    public Object onHello { get; set; }

    public String Hello(string name)
    {
        var r = FireEvent();
        return "Why, Hello, " + name + "!!!" + r;
    }
}

The main trick is the FireEvent method. This worked for me.

    private string FireEvent()
    {
        if (onHello == null) return " (N/A)";
        onHello
            .GetType()
            .InvokeMember
            ("",
             BindingFlags.InvokeMethod,
             null,
             onHello,
             new object [] {});

        return "ok";
    }

Compile that all together, register it with regasm:

%NET64%\regasm.exe Cheeso.Greet.dll /register /codebase

…And then use it from JScript like this:

var greet = new ActiveXObject("Cheeso.Greet"), response;
greet.onHello = function() {
    WScript.Echo("onHello (Javascript) invoked.");
};
response = greet.Hello("Fred");
WScript.Echo("response: " + response);

It works.

You can also call it from VBScript:

Sub onHello ()
    WScript.Echo("onHello (VBScript) invoked.")
End Sub

Dim greet
Set greet = WScript.CreateObject("Cheeso.Greet")
greet.onHello = GetRef("onHello")
Dim response
response = greet.Hello("Louise")
WScript.Echo("response: " &  response)

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To pass parameters back from C# to JScript with this approach, I think objects need to be IDispatch, but of course you can send back simple values marshaled as string, int, and so on which are marshaled as you would expect.

For example, modify the C# code to send back a reference to itself, and the number 42.

        onHello
            .GetType()
            .InvokeMember
            ("",
             BindingFlags.InvokeMethod,
             null,
             onHello,
             new object [] { this, 42 });

Then, you can get that in jscript like so:

greet.onHello = function(arg, num) {
    WScript.Echo("onHello (Javascript) invoked.");
    WScript.Echo("  num = " + num + "  stat=" + arg.status);
};

Or in VBScript like so:

Sub onHello (obj, num)
    WScript.Echo("onHello (VBScript) invoked. status=" & obj.status )
    WScript.Echo("  num= " & num)
End Sub

NB: You can define your jscript event handler function to accept fewer args than are sent by the C# object when invoking the “event”. In my experience you need to design the event handler in VBScript to explicitly accept the correct number of arguments.