Passing JSON objects to and from C#

[AndyHit5687]

Hi All, I am new to "playing" with CefSharp so am feeling my way. I have successfully embedded it within a WinForm application. I have called a C# method from JavaScript and invoked a delayed callback in JS. I was trying to pass a JSON object into the C# method and added a parameter of type Object to receive it so I could determine the type, it arrives and "GetType()" returns Dictionary'2... I guessed at Dictionary<string,string> but this results in an invalid cast. Can anyone tell me the correct type to use? I would also like to return a value that JS would see as a JSON object.

Also, is there a "promise" equivalent to "IJavascriptCallback"?

public void TestCallback(IJavascriptCallback javascriptCallback, object param)

[amaitland]

I was trying to pass a JSON object into the C# method and added a parameter of type Object

If you passed an object to a method as a parameter then there's no JSON involved.

The default type for an object is https://docs.microsoft.com/en-us/dotnet/api/system.dynamic.expandoobject?view=net-5.0 Which you can cast to System.Collections.Generic.IDictionary<string,object> should you choose to. You can use the dynamic keyboard with ExpandoObject.

Rather than describe code in future please provide examples of both your JavaScript and c#.

I would also like to return a value that JS would see as a JSON object.

You can return a POCO and you will get an object in JavaScript. Again no JSON is actually used.

Also, is there a "promise" equivalent to "IJavascriptCallback"?

No. What exactly are you trying to achieve?

[AndyHit5687]

I will do that. I need a little time to get to the point where I have a working example as I need to learn and resolve things as I go. I will concentrate on the data transfer link between JS and C# and just stub out the actual background data retrieval etc.
Is there a better place to put the example code rather than paste or attach it here or is this an appropriate place?

Thanks Andy H

[AndyHit5687]

Alex, just discovered that I can call a JS function with a nested Dictionary<string, object> but cannot return the same. Returning a dictionary raises an exception in "TypeUtils.cpp". Is this by design or omission? If it is the latter would it help if I pulled the latest version of CefSharp and tried to make any necessary changes myself and then sent them back to you for review and hopefully inclusion in the source code? If the latter are there any instructions on how to set up the VS2019 project, that is do I need to download anything extra such as Chrome API stuff in order to compile and link or is CefSharp self contained?

The method below successfully calls my callback and the Chrome debugger sees a JS object like I would expect, even the nested dictionary but changing the return from null to include the same format dictionary raises an exception saying unable to convert. I think this is the line:

//TODO: What exception type?
throw gcnew Exception("Cannot convert object from Cef to CLR.");

    public Dictionary<string, object> TestCallback(IJavascriptCallback javascriptCallback, object param)
    {
        Dictionary<string, object> result = new Dictionary<string, object>
        {
            { "One", "Fluffy" },
            { "Two", 1 } //,
            //{ "Three", new Dictionary<string, object>
            //    {
            //        {"A", 1},
            //        {"B", 2},
            //    }
            //}
        };


        const int taskDelay = 1500;
        IDictionary<string, object> paramObject = (IDictionary<string, object>)param;

        Task.Run(async () =>
        {
            await Task.Delay(taskDelay);

            using (javascriptCallback)
            {
                Dictionary<string, object> result2 = new Dictionary<string, object>
                {
                    { "X", "John" },
                    { "Y", 42 },
                    { "Z", new Dictionary<string, object>
                        {
                            {"A", 1},
                            {"B", 2}
                        }
                    }
                };

                await javascriptCallback.ExecuteAsync("This callback from C# was delayed " + taskDelay + "ms", result2); //<<== succeeds and JS object created.
            }
        });
        return null; // result; // <<= raises exception in TypeUtils.cpp saying no convertion.
    }

[amaitland]

The Async JavaScript Binding implementation should support nested dictionaries.

https://github.com/cefsharp/CefSharp/blob/cefsharp/87/CefSharp.Example/JavascriptBinding/AsyncBoundObject.cs#L179

The older legacy synchronous JavaScript Binding is not something I have plans to add new features to as per https://github.com/cefsharp/CefSharp/wiki/General-Usage#sync-javascript-binding-jsb

If the change is minimal and includes appropriate qunit tests then I'd consider a PR.

If you use a POCO then nested properties should already be supported.

CefSharp self contained?

Nuget will download the required dependencies. The project uses the Chromium embedded framework, see https://github.com/cefsharp/CefSharp/wiki/General-Usage for background. Chrome doesn't expose an API.

If you need to use the debugger see https://www.chromium.org/developers/how-tos/debugging-on-windows specially the Manually attaching to a child process section.

Pick the x86 platform from the menu in VS.

[AndyHit5687]

Alex,

I modified my C# object registration to:

        m_chromeBrowser.JavascriptObjectRepository.Settings.LegacyBindingEnabled = true;
        m_chromeBrowser.JavascriptObjectRepository.Register("ReportController", new ReportController(m_chromeBrowser, this), isAsync: true, options: BindingOptions.DefaultBinder);

And re-instated the return of a nested dictionary object. No exception now so good. Using the chrome debugger I discovered that JS call to my C# function returned a "promise" so I changed the JS code to this just to try it out.

    function InstigateCallback() {
        var returnResult;

// returnResult = ReportController.testCallback(MyCallback, person);
returnResult = ReportController.testCallback(MyCallback, person);
returnResult.then(function (returnObject) {
alert("THEN");
}).catch(function () {
alert("CATCH");
});
}
function MyCallback(msg, result) {
alert("My callback returned - " + msg + " X = " + result.X + " Y = " + result.Y);
}
where the "then" function parameter "returnObject" is a JS object that represents the dictionary returned from C#.

I don't know where to look but I assume you have built in some promise support. I then added a Thread.Sleep(10000) into my C# code to stub a lengthy operation before returning and placed another alert after the promise code:

    function InstigateCallback() {
        var returnResult;

// returnResult = ReportController.testCallback(MyCallback, person);
returnResult = ReportController.testCallback(MyCallback, person);
returnResult.then(function (returnObject) {
alert("THEN");
}).catch(function () {
alert("CATCH");
});
alert("XXX");
}
function MyCallback(msg, result) {
alert("My callback returned - " + msg + " X = " + result.X + " Y = " + result.Y);
}

And the alerts occurred "XX" -> "My callback..." -> "THEN" which is what I wanted. I haven't tried making another C# call before the promise is fulfilled.

[amaitland]

Also, is there a "promise" equivalent to "IJavascriptCallback"?

In version 88.2.90 I've added manual support for resolving a promise when calling IJavascriptCallback.ExecuteAsync. It's a manual process and requires some extra handling, I've added an example to
https://github.com/cefsharp/CefSharp/wiki/Advanced-Async-JavaScript-Binding-(JSB)#javascript-callback-with-promise