如何在c中设置html元素的值

我正在开发一个从网站获取数据的项目。所以我使用了webBrowser并将url设置为travelchi.ir，我在下面写了代码

 private void webBrowser1_DocumentCompleted(object sender, WebBrowserDocumentCompletedEventArgs e)
        {
            HtmlDocument doc = webBrowser1.Document;
            HtmlElement Source = doc.GetElementById("from");
            HtmlElement Destination = doc.GetElementById("to");
            HtmlElement adultCount = doc.GetElementById("adultCount");
            HtmlElement childCount = doc.GetElementById("childCount");
            HtmlElement infantCount = doc.GetElementById("infantCount");
            var links = webBrowser1.Document.GetElementsByTagName("button");
            var inputs = webBrowser1.Document.GetElementsByTagName("input");
            Source.SetAttribute("value", "شیراز");
            Destination.SetAttribute("value", "تهران");
            adultCount.SetAttribute("value", "1 بزرگسال");
            childCount.SetAttribute("value", "1 کودک");
            infantCount.SetAttribute("value", "0 خردسال");
            foreach (HtmlElement input in inputs)
            {
                if (input.GetAttribute("className") == "form-control")
                {
                    input.SetAttribute("value", "1395/05/14");
                }
            }
            foreach (HtmlElement link in links)
            {
                if (link.GetAttribute("className") == "btn btn-primary")
                {
                    link.InvokeMember("click");
                }
            } 

要获取元素并为其设置值，所以在最后我发现此代码无法设置值，请帮助我解决