无效尝试接入领域调用read（）之前 [英] Invalid attempt to access field before calling read()

问题描述

我已经寻找一个好几个小时了，寻找一个解决这个问题。我试图让使用下面用正确的查询代码从我的数据库信息。我知道这些查询工作，因为我已经在MySQL的工作台进行了测试。我不断收到错误：

无效的尝试调用read（）

之前访问场

正如你所看到的，我不调用read（），而我的研究才想出没有使用正确的数据库，我也证实是正确的了。任何想法，为什么我收到这个错误？在此先感谢您的帮助。

 公共静态字符串的ExecuteSelect（查询字符串）
 {
 //示例查询是：SELECT ENTITY_ID FROM catalog_product_flat_1 WHERE SKU ='itemSku'; 
字符串声明=; 
 
的MySqlCommand myCommand =新的MySqlCommand（查询，_conn）; 
 
了MySqlDataReader myReader; 
 myReader = myCommand.ExecuteReader（）; 
 myReader.Read（）; 
 
语句= myReader.GetString（0）; 
 myReader.Close（）; 
 
 return语句; 
} 
  

解决方案

不知道这是你的情况的问题，但您应经常检查阅读（）的结果。例如：

 如果（myReader.Read（））
 {
语句= myReader.GetString（0） ; 
} 
  

修改同时你实际上是在做什么检索标量，因此你可以使用的ExecuteScalar（）

 收益率（myCommand.ExecuteScalar（）??的String.Empty）的ToString（）; 
 //也重新命名你的方法正确
  

I have searched for a good couple hours now, looking for a solution to this problem. I am trying to get information from my database using the code below with the correct queries. I know these queries work because I have tested them in MySQL workbench. I keep getting the error:

Invalid attempt to access field before calling read()

As you will see, I do call read(), and my research has only come up with not using the correct database, which I have also verified to be correct already. Any ideas as to why I am receiving this error? Thanks in advance for the help.

public static string ExecuteSelect(string query)
{
    //Example query is: SELECT entity_id FROM catalog_product_flat_1 WHERE sku='itemSku';
    string statement = "";

    MySqlCommand myCommand = new MySqlCommand(query, _conn);

    MySqlDataReader myReader;
    myReader = myCommand.ExecuteReader();
    myReader.Read();

    statement = myReader.GetString(0);
    myReader.Close();

    return statement;
}

解决方案

Not sure if this is the problem in your case, but you should always check the result of Read(). eg

if (myReader.Read())
{
  statement = myReader.GetString(0);
}

Edit: Also what you are actually doing is retrieving a scalar, and as such you could use ExecuteScalar()

return (myCommand.ExecuteScalar() ?? string.Empty).ToString();
//also rename your method appropriately

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