MessageBox.Show("Message");
To display Alert Box, use
System.Windows.Browser.HtmlPage.Window.Alert("Alert Message");
To display Input box (prompt), use
string strValue = System.Windows.Browser.HtmlPage.Window.Prompt("What is your age?");
To display confirm box, use
bool blnResult = System.Windows.Browser.HtmlPage.Window.Confirm("Are you sure?");