codice:
...
<input type="text" id="TextBox" value="textBox" />
...
...
<form id="form1" runat="server" style="height:100%">
<asp:TextBox ID="TextBox1" runat="server" Text = "textBox1"></asp:TextBox>
</form>
...
...
HtmlElement textBox = HtmlPage.Document.Body.Children.OfType<HtmlElement>().Single(element => element.Id == "TextBox");
var text = textBox.GetProperty("value");
textBox.SetProperty("value","Nuovo testo");
HtmlElement form1 = HtmlPage.Document.Body.Children.OfType<HtmlElement>().Single(form => form.Id == "form1");
HtmlElement textBox1 = form1.Children.OfType <HtmlElement>().Single(element => element.Id == "TextBox1");
var text1 = textBox1.GetProperty("value");
textBox1.SetProperty("value", "Nuovo testo 1");
...
ovviamente devi considerare i vari annidamenti partendo dalla radice.