Why does IHTMLImgElement method put_src returns E_OUTOFMEMORY when called from thread?

Question

I have a BHO(Internet Explorer extension) and I'm trying to set a "data:image/png;base64, [code]" (base64 encoded image buffer) as the src of an IHTMLImgElement. When put_src was called from the main thread of the BHO, there was no problem, but now that I call it from a different thread, it returns E_OUTOFMEMORY. The base64 encoded string is very long. For a shorter base64 string it returns no error.

I have to specify that I have no memory leaks, I used only smart COM pointers, and deleted all allocated memory.

Also I use the same type of base64 string to call set_backgroundImage for IHTMLStyle objects and it returns no error.

Answer 1

If the IWebBrowser2 object is marshaled to another thread the string passed to put_src method must be a BSTR allocated with SysAllocString, not a simple LPWSTR. I got the answer from another forum.

Answer 2

What does it specifically mean "not thread safe"? As I wrote in the main post the call to set_backgroundImage returns with no problem.

In this context, it simply means you cannot call it directly from any other thread but the main thread, where your BHO was created. If you need to use it from another thread, marshal the object's IHTMLImgElement interface to that thread, e.g. with CoMarshalInterThreadInterfaceInStream.

More info:

• How to marshal interfaces across apartments in Visual C++

• Descriptions and Workings of OLE Threading Models