如何使用查询字符串，如下所示

Here is my code

这是我的代码

var SearchUser = (from u in db.Users
                  join l in db.LoggedInUsers on u.UserID equals l.UserID
                  where u.Username==txtSearch.Text
                  select u).SingleOrDefault();

litUsers.Text = "<div style='background-color:#B0C4DE;color:red'>"
                + "<a href=\"ChatWindow.aspx\">" + SearchUser.Username + "</a></div>";
var SearchUser = (fr