Manipulating HTML with Fiddler

fiddler-html-agility-pack.cs

using Fiddler;
using HtmlAgilityPack;
using System.Collections.Generic;
using System.IO;
using System.Linq;

public class LinkAutoTamper : IAutoTamper2
{
    public void AutoTamperRequestAfter(Session oSession)
    {
    }

    public void AutoTamperRequestBefore(Session oSession)
    {
    }

    public void AutoTamperResponseAfter(Session oSession)
    {
    }

    public void AutoTamperResponseBefore(Session oSession)
    {
        if (!oSession.ResponseHeaders.ExistsAndContains("Content-Type", "text/html"))
        {
            return;
        }
        var body = oSession.GetResponseBodyAsString();
        var encoding = oSession.GetResponseBodyEncoding();
        var doc = new HtmlDocument();
        doc.LoadHtml(body);
        var anchors = doc.DocumentNode?.SelectNodes("//a")?.ToList() ?? new List<HtmlNode>();
        var httpAnchors = anchors.Where(node => node.GetAttributeValue("href", "").OICStartsWith("http:"));
        foreach(var node in httpAnchors)
        {
            node.SetAttributeValue("style", node.GetAttributeValue("style", "") + "; border: 1px solid red");
        }
        using (var ms = new MemoryStream())
        {
            doc.Save(ms, encoding);
            oSession.ResponseBody = ms.ToArray();
        }
    }

    public void OnBeforeReturningError(Session oSession)
    {
    }

    public void OnBeforeUnload()
    {
    }

    public void OnLoad()
    {
    }

    public void OnPeekAtResponseHeaders(Session oSession)
    {
        if (oSession.ResponseHeaders.ExistsAndContains("Content-Type", "text/html"))
        {
            oSession.bBufferResponse = true;
        }
    }
}

fiddler-html-agility-pack.fs

open Fiddler
open HtmlAgilityPack
open System.IO

type LinkAutoTamper() =
    interface IAutoTamper2 with
        member this.AutoTamperRequestAfter(_) = ()
        member this.AutoTamperRequestBefore(_) = ()
        member this.AutoTamperResponseAfter(_) = ()
        member this.AutoTamperResponseBefore(session) = 
            if not <| session.ResponseHeaders.ExistsAndContains("Content-Type", "text/html") then
                ()
            else
                let body = session.GetResponseBodyAsString()
                let encoding = session.GetResponseBodyEncoding()
                let doc = HtmlDocument()
                doc.LoadHtml(body)
                let anchors : HtmlNode list = 
                    match doc.DocumentNode with
                    | null -> []
                    | docNode -> match docNode.SelectNodes("//a") with | null -> [] | a -> a |> Seq.toList
                
                anchors
                |> Seq.filter(fun node -> node.GetAttributeValue("href", "").OICStartsWith("http:"))
                |> Seq.iter(fun node ->
                    node.SetAttributeValue("style", node.GetAttributeValue("style", "") + "; border: 1px solid red")
                    |> ignore)
                use ms = new MemoryStream()
                doc.Save(ms, encoding)
                session.ResponseBody <- ms.ToArray()
                ()

        member this.OnPeekAtResponseHeaders(session) =
            if session.ResponseHeaders.ExistsAndContains("Content-Type", "text/html") then
                session.bBufferResponse <- true

        member this.OnBeforeReturningError(_) = ()
        member this.OnLoad() = ()
        member this.OnBeforeUnload() = ()