How to load CanvasSvgDocument from file? - c++-winrt

Question

In a C++/winrt project I have a large number of small svg resources to be loaded from file. Since it would be slow to reload them all from disk at each CreateResources event from the CanvasVirtualControl I have loaded them in advance and stored the data for each in an array. When CreateResources happens my intent is to load a CanvasSvgDocument for each of these by using the CanvasSvgDocument method LoadFromXml(System.string). However, If I create an svgDocument using the resourcecreator, I get an invalid argument crash when calling LoadFromXml(). The resourceCreator argument looks right (VS preview 6 now allows me to see local variables!) and the xml data string argument looks like the valid svg data, so my best guess about the crash is that the data string is the wrong format. The file data is UTF-8. If I convert that to a std::wstring as I must for the LoadFromXml argument can it still be understood as byte data?
For example, I create the std::wstring this way, given a pointer to unsigned char file data and its length in bytes:
m_data_string = std::wstring(data, data + dataLength);
When CreateResources is triggered that datastring is referenced this way:
m_svg = CanvasSvgDocument(resourceCreator);
m_svg.LoadFromXml(resourceCreator, m_data_string);
But LoadFromXml crashes with that invalid parameter error. I see that the length of the data string is correct, but of course that is the number of characters, not the actual size of the data. Could there be a conflict between the UTF-8 attribute in the svg and the fact that it is now recorded as 16-bit characters? If so, how would one load an xml document from such data?
[Update] with the suggestion that I use winrt::to_hstring. I read the unsigned char data into a std::string,
std::string cstring = std::string("");
cstring.assign(data, data + dataLength);
Then I convert that:
m_data_string = winrt::to_hstring(cstring);
And finally try to load an svg as before:
m_svg.LoadFromXml(resourceCreator, m_data_string);
And it crashes as before. I notice that in the debugger that converted string in neither case appeared to be gibberish - in both cases it read in the debugger as the expected svg data. But if this hstring is wide chars wouldn't that be a conflict with the attribute in the svg that identifies it as UTF-8?
[Update] I'm starting to wonder if anyone has ever used CanvasSvgDocument.Draw() to draw an svg loaded from a file. The files are now loading without crashing without any change to their internal encoding reference. But - they won't draw. These files - 239 of them - are UTF-8, svg 1.1, and they display nicely if opened in Edge or any browser. But if I load the file data to an hstring, create a CanvasSvgDocument and then use CanvasSvgDocument.LoadFromXml to load them, they do not draw when called by CanvasSvgDocument's draw method. Other drawing of shapes, etc. works fine during the drawing session. Here is what could be a hint: If I call GetXML() on one of these svgs after it is loaded, what is returned is just this:
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"></svg>
That is, the drawing information is not there. Or is this the full extent of what GetXml() is meant to return? That wouldn't seem useful. So perhaps CanvasSvgDocument.LoadFromXml(ResourceCreator, String) doesn't actually work yet?
So I'm back to asking again: is there a way to load a functional CanvasSvgDocument from file data?

Answer 1

My first answer here was wrong: the fault in my code above is that LoadFromXml() is a static method, and someone pointed out to me elsewhere, I was discarding the returned result. It should be theSvg = CanvasSvgDocument::LoadFromXml(string).
Having corrected that, I'm back to the problem of loading UTF-8 data in a method whose argument is a wide-character string. Changing the internal reference to UTF-16 doesn't help after all. Loading the svg with CanvasSvgDocument::LoadAsync(filestream) works, but if I want to load these without re-accessing the disk I will need to find a way to make RandomAccessFileStream from a buffer of bytes and then use LoadAsync. I think. Unless there is some other way to make LoadFromXmL() work - at present it fails with an invalid argument error.