How to find and replace href values on links using AngleSharp?

The great thing about AngleSharp is that you can basically use any JS solution - because AngleSharp exposes the W3C DOM API (JS uses it too). All you need to do is replace some camelCase with PascalCase and use standard .NET tools instead of the ones in JS.

Let's take " how to change all links using javascript" as an example (sorry it was the first hit of my Google search) and use that as a starting point.

var context = BrowsingContext.New(Configuration.Default);
var document = await context.OpenAsync(res => res.Content(""));
var anchors = document.GetElementsByTagName("a");

for (var i = 0; i < anchors.Length; i++)
{
    var anchor = anchors[i] as IHtmlAnchorElement;
    anchor.Href = "http://example.com/?redirect=" + anchor.Href;
}

So in our case we are not interested in the same transformation, but very interested in similar transformations. we can do it:

for (var i = 0; i < anchors.Length; i++)
{
    var anchor = anchors[i] as IHtmlAnchorElement;

    if (anchor.GetAttribute("href")?.StartsWith("#") ?? false)
    {
        anchor.Href = "your-new-value";
    }
}

The reason is that Hrefit is always normalized (i.e. the full URL) so that the attribute value of "#foo" looks like "http://example.com/path#foo". By looking at the original value, we can just assume that the value still starts with the hash symbol.