c# - 从MySQL表接收DownloaLink

我尝试在MySQL表中搜索ID，然后返回包含URL的关联字符串列。
但是下载器总是告诉我链接格式是错误的。

这是我的方法：

      public string URL(string ID)
    {
        MySqlConnection con = new MySqlConnection("host=localhost;user=root;password=root;database=Downloader;");
        MySqlCommand cmd = new MySqlCommand("SELECT string FROM Files WHERE ID = '" + ID + "';");
        cmd.Connection = con;
        con.Open();
        MySqlDataReader reader = cmd.ExecuteReader();
        return reader.ToString();
    }

这里是下载器：

 private void button1_Click(object sender, EventArgs e)
    {
        sql_reader sql_reader = new sql_reader();
      if (!Directory.Exists("Downloads"));
        {
            Directory.CreateDirectory("Downloads");
        }

        WebClient client = new WebClient();
    client.DownloadProgressChanged += new DownloadProgressChangedEventHandler(client_DownloadProgressChanged);
    client.DownloadFileCompleted += new AsyncCompletedEventHandler(client_DownloadFileCompleted);
    client.DownloadFileAsync(new Uri(sql_reader.URL(textBox1.Text)), "./Downloads/" + textBox2.Text + "." + textBox3.Text);

textBox1是我在其中输入ID的Box。

最佳答案

1.您需要先调用Read()函数，然后再从MySqlDataReader对象获取记录

2.您必须在访问记录时指定column name。

例：

            String result="";
            MySqlDataReader reader = cmd.ExecuteReader();
            if(reader.Read())
            result=reader["columnname"].ToString();

3.我建议您使用Parametrised Queries来避免SQL injection Attacks。

例：

MySqlCommand cmd = new MySqlCommand("SELECT string FROM Files WHERE ID =@ID;");
cmd.Parameters.AddWithValue("@ID",ID);

4.正确dipose MySqlCommand，MySqlConnection和MySqlDataReader对象。您可以使用using{}块将它们干净地处置。

完整的代码

 public string URL(string ID)
        {
            String result="";
            using(MySqlConnection con = new MySqlConnection("host=localhost;user=root;password=root;database=Downloader;"))
           {
            using(MySqlCommand cmd = new MySqlCommand("SELECT string FROM Files WHERE ID =@ID;"))
            {
            cmd.Connection = con;
            con.Open();
            cmd.Parameters.AddWithValue("@ID",ID);
            using(MySqlDataReader reader = cmd.ExecuteReader())
            {
            if(reader.Read())
            result=reader["string"].ToString();
            else
            result="";//you can customize here
            }
           }
          }
            return result;
        }

解决方案2：

替换此语句：

client.DownloadFileAsync(new Uri(sql_reader.URL(textBox1.Text)), "./Downloads/" + textBox2.Text + "." + textBox3.Text);

有了这个：

client.DownloadFileAsync(new Uri(sql_reader.URL(textBox1.Text),UriKind.Absolute), "./Downloads/" + textBox2.Text + "." + textBox3.Text);