IWebbrowser2: Using a Uint8array (filling without looping)

I can create a Uint8array in an IWebbrowser2 window:

IHTMLWindow2 window = ...;
DISPID dispid_uint8array = ...;

VARIANT self;
self.vt = VT_NULL;

VARIANT length;
length.vt = VT_I4;
length.lVal = 100;

VARIANT args[2] = { self, length };
DISPID named_args[1] = { DISPID_THIS };

DISPPARAMS params;
params.rgvarg = args;
params.rgdispidNamedArgs = named_args;
params.cArgs = 2;
params.cNamedArgs = 1;

VARIANT result;
result.vt = VT_EMPTY; 
HRESULT hr = container->Invoke(
    dispid_uint8array, IID_NULL, LOCALE_USER_DEFAULT,
    DISPATCH_METHOD, &params, &result, nullptr, nullptr
);

assert(hr == S_OK);
assert(result.vt == VT_DISPATCH);

IDispatch my_new_uint8array = result.pdispVal;

Now I can set items fields of my_new_uint8array using IDispatch::Invoke(..., DISPATCH_PROPERTYPUT, ...) in a loop.

But isn't there a proper proper interface to fill the whole buffer with just one call? E.g. can I somehow retrieve the contained buffer?

Is there a definition of the interface somewhere, something like IUint8Array?

Solution

I came up with an incredibly ugly hack: Using the ArrayBuffer of a <canvas>. I'll only post the pseudo code, because the full C++, including error checking, unwrapping etc, is just too long:

Prelude:

FEATURE_BROWSER_EMULATION was set to 11001
IWebBrowser2::get_ReadyState returns READYSTATE_COMPLETE
IHTMLDocument6::get_documentMode returns 11
IHTMLDocument5::get_compatMode returns "CSS1Compat"
IHTMLDocument2 *doc and IHTMLWindow2 *win are set.
I use simple strings in my pseudo code, but you have to use BSTRs of course!
You might have to queryInterface once or twice, so that the results fit.
Don't ignore errors!

How to get a "Uint8Array":

ULONG len_in_dwords = (LENGTH_I_WANT + 3) / 4;

IHTMLCanvasElement *canvas = doc->createElement("canvas");
ICanvasRenderingContext2D *context = canvas->getContext("2d");
ICanvasPixelArrayData *array_data = context->createImageData(len_in_dwords, 1);

// use this variable for interfaces that accept an ArrayBuffer
IDispatch *array_buffer = Get property "buffer" of array_data;

// use this variable to edit the content:
BYTE *byte_buffer;
ULONG buffer_length_in_bytes;
array_data->GetBufferPointer(&byte_buffer, &buffer_length_in_bytes);

// no need for that anymore:
canvas->Release();
context->Release();
array_data->Release();

The ArrayBuffer array_buffer has always a size divisible by four. That works for me, but might not work for other use cases. You can use array_buffer's method slice(0, LENGTH_I_WANT) to remove the extra bytes after memcpy'ing to byte_buffer.