NameValueCollection nvc = Request.Form;
string userName, password;
if (!string.IsNullOrEmpty(nvc["txtUserName"]))
{
  userName = nvc["txtUserName"];
}

if (!string.IsNullOrEmpty(nvc["txtPassword"]))
{
  password = nvc["txtPassword"];
}

//Process login
CheckLogin(userName, password);

if (!string.IsNullOrEmpty(Request.Form["username"]) { ... }

NameValueCollection nvc = Request.Form;
string userName, password;
    if (!string.IsNullOrEmpty(nvc["txtUserName"]))
    {
      userName = nvc["txtUserName"];
    }

    if (!string.IsNullOrEmpty(nvc["txtPassword"]))
    {
      password = nvc["txtPassword"];
    }

    //Process login
    CheckLogin(userName, password);