如何检测C#中是否加载了Iframe

using System;
using System.Windows.Forms;

namespace IframeDetection
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            webBrowser1.DocumentCompleted += WebBrowser1_DocumentCompleted;
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            webBrowser1.Navigate("https://www.example.com");
        }

        private void WebBrowser1_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
        {
            HtmlDocument document = webBrowser1.Document;
            HtmlElement iframe = document.GetElementById("iframeId"); // 替换为实际的iframe元素id
            if (iframe != null)
            {
                // iframe加载完成
                MessageBox.Show("Iframe loaded");
            }
            else
            {
                // iframe未加载
                MessageBox.Show("Iframe not loaded");
            }
        }
    }
}