Mutation Observer throws when connecting to a document

Bug Report

Prerequisites

• [y] Can you reproduce the problem in a MWE?
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Description

I’m trying to use a mutation observer to watch an entire document. Unfortunately when I try to do that I get a null reference exception.

Steps to Reproduce

var document = ... //from somewhere
var observer = new MutationObserver((mutations, observer) => {});
observer.Connect(document);

Expected behavior: no exception should be thrown

Actual behavior: a null reference exception was thrown when the connect method tries to access the document owner which is always null

Environment details: Windows .NET 5

Possible Solution

In this file: https://github.com/AngleSharp/AngleSharp/blob/devel/src/AngleSharp/Dom/MutationObserver.cs#L221 the following line in the issue

node.Owner.Mutations.Register(this);

the problem is that a document always returns null for it’s owner. I see 2 ways to fix this, the first is to change the document so returns itself as it’s owner. The other way would be to refactor this line to check if the node is a document and if it is then just use the Mutations Property directly. Something like this

if (node is Document d)
    d.Mutations.Register(this);
else
    node.Owner.Mutations.Register(this);

[FlorianRappl]

Fix released in preview https://www.nuget.org/packages/AngleSharp/0.16.1-alpha-120.

[hahn-kev]

I’m sorry, I think my example was incorrect, instead of this: observer.Connect(document); it should have been this observer.Connect(document, true); so I wouldn’t get an error about any of those settings being false.

I did a little testing (in Firefox and Chrome) and it seems perfectly happy to observe the document without any errors. If you want to test it out copy this into your browser console and modify something in the html.

// Options for the observer (which mutations to observe)
const config = { attributes: true, childList: true, subtree: true };

// Callback function to execute when mutations are observed
const callback = function(mutationsList, observer) {
    // Use traditional 'for loops' for IE 11
    for(const mutation of mutationsList) {
        if (mutation.type === 'childList') {
            console.log('A child node has been added or removed.');
        }
        else if (mutation.type === 'attributes') {
            console.log('The ' + mutation.attributeName + ' attribute was modified.');
        }
    }
};

// Create an observer instance linked to the callback function
const observer = new MutationObserver(callback);

// Start observing the target node for configured mutations
observer.observe(document, config);

Like you said a document is a node, in AngleSharp and the browser, there’s just a minor detail in AngleSharp that seems to prevent it from working currently. As for step 3 of the algorithm I don’t see what this has to do with a document not being accepted.