为什么访问不了SQL数据库 - .NET技术 / C#

public static DataSet ExecuteDataSet(string StrSql)
  {
  SqlConnection strcon = new SqlConnection(con());
  try
  {
  SqlCommand cmd = new SqlCommand(StrSql, strcon);
  cmd.CommandTimeout = 180;
  cmd.CommandText = StrSql;
  cmd.CommandType = CommandType.Text;
  SqlDataAdapter sda = new SqlDataAdapter();
  sda.SelectCommand = cmd;
  DataSet ds = new DataSet();
  sda.Fill(ds);
  sda.Dispose();
  return ds;
  }
  catch  
  {

  if (strcon.State == ConnectionState.Open)
  strcon.Close();
  strcon.Dispose();
  return null;
  }
  finally
  {
  strcon.Dispose();
  if (strcon.State == ConnectionState.Open)
  strcon.Close();
  }
  }

登录的时候 查询 用户名和密码  
  sb.Append(" Select * from Table_d Where [LoginUser]='" + _LoginUser + "' and [LoginPwd]='" + _LoginPwd + "'");

  DataSet _Login_Ds = DbHelper.ExecuteDataSet(sb.ToString());
  if (_Login_Ds != null)
  {
  DataTable _Login_Dt = _Login_Ds.Tables[0];
  if (_Login_Dt.Rows.Count > 0)
  {
Session["Department"] = _Login_Dt.Rows[0]["Department"].ToStri