C# 在调用Read()之前获取访问字段的无效尝试,但在C中调用了Read()#
好的,我正在尝试使用MySQL connector 6.9.5通过C#从MySQL获取数据,我得到以下错误消息:C# 在调用Read()之前获取访问字段的无效尝试,但在C中调用了Read()#,c#,mysql,C#,Mysql,好的,我正在尝试使用MySQL connector 6.9.5通过C#从MySQL获取数据,我得到以下错误消息: Error: MySql.Data.MySqlClient.MySqlException: Invalid attempt to access a field before callingRead() at MySql.Data.MySqlClient.ResultSet.get_Item(Int32 index) at MySql.Data.MySqlClient.MySqlDat
Error: MySql.Data.MySqlClient.MySqlException: Invalid attempt to access a field before callingRead()
at MySql.Data.MySqlClient.ResultSet.get_Item(Int32 index)
at MySql.Data.MySqlClient.MySqlDataReader.GetFieldValue(Int32 index, Boolean checkNull)
at MySql.Data.MySqlClient.MySqlDataReader.GetString(Int32 i)
at PayrollManagementSystem.ConnectorDB.login(String username, String password) in C:\Users\Scarlet\Documents\Visual Studio 2008\Projects\PayrollManagementSystem\PayrollManagementSystem\ConnectorDB.cs:line 41
这意味着我试图在调用Read()方法之前访问一个字段,但我想我已经访问了,下面是我的代码:
public ModelUser login(String username, String password)
{
Console.WriteLine("username asal = " + username);
ModelUser val = null ;
connection.ConnectionString = connectionString;
try
{
command = new MySqlCommand( "SELECT user.id_data_user, user.username, user.password FROM user WHERE username = '" + username + "'" +
" AND password = '" + password + "'", connection);
MySqlDataReader reader;
connection.Open();
reader = command.ExecuteReader();
val = new ModelUser();
reader.Read(); == this is the Read() method was called
val.IDDataUser = reader.GetString(0); === error at this line
val.Username = reader.GetString(1);
val.Password = reader.GetString(2);
Console.WriteLine("data_user di conDb: " + val.IDDataUser);
connection.Close();
}
catch (MySqlException ex)
{
Console.WriteLine("Error: {0}", ex.ToString());
}
return val;
}
我不知道我的代码出了什么问题,这是我第一次尝试C#,非常感谢您对我的问题的回复
更新:
我像这样修改了代码
if (reader.HasRows)
{
while (reader.Read())
{
val.IDDataUser = reader.GetString(0);
val.Username = reader.GetString(1);
val.Password = reader.GetString(2);
}
}
else
{
Console.WriteLine("Holy cow,she's got no rows!");
}
我的读者没有意见。就像它无法从数据库中获取任何数据一样。这就是日志的样子:
'PayrollManagementSystem.vshost.exe' (Managed (v2.0.50727)): Loaded 'C:\Windows\assembly\GAC_MSIL\System.Configuration\2.0.0.0__b03f5f7f11d50a3a\System.Configuration.dl
', Skipped loading symbols. Module is optimized and the debugger option 'Just My Code' is enabled.
'PayrollManagementSystem.vshost.exe' (Managed (v2.0.50727)): Loaded 'C:\Windows\assembly\GAC_64\System.EnterpriseServices\2.0.0.0__b03f5f7f11d50a3a\System.EnterpriseServices.dll', Skipped loading symbols. Module is optimized and the debugger option 'Just My Code' is enabled.
Holy cow,she's got no rows!
username:
data user:
id :
The thread '<No Name>' (0x13f0) has exited with code 0 (0x0).
'PayrollManagementSystem.vshost.exe'(托管(v2.0.50727)):加载的'C:\Windows\assembly\GAC\u MSIL\System.Configuration\2.0.0\uu b03f5f7f11d50a3a\System.Configuration.dl
,已跳过加载符号。模块已优化,并且调试器选项“仅我的代码”已启用。
“PayrollManagementSystem.vshost.exe”(托管(v2.0.50727)):加载的“C:\Windows\assembly\GAC\U 64\System.EnterpriseServices\2.0.0\Uuu b03f5f7f11d50a3a\System.EnterpriseServices.dll”跳过了加载符号。模块已优化,并且调试器选项“仅我的代码”已启用。
天哪,她没有吵架!
用户名:
数据用户:
身份证件:
线程“”(0x13f0)已退出,代码为0(0x0)。
您需要检查读卡器
是否读取一行
使用数据读取器的最佳方法是
if(reader.HasRows)
{
while(reader.Read()) // == this is the Read() method was called
{
val.IDDataUser = reader.GetString(0); === error at this line
val.Username = reader.GetString(1);
val.Password = reader.GetString(2);
Console.WriteLine("data_user di conDb: " + val.IDDataUser);
}
}
查看msdn参考,我还建议/建议更改该查询以利用
参数将代码放入while循环while(reader.Read()){}
我尝试过,但读取器没有行,我的查询是对的。@Yusuf1494如果(reader.HasRows)检查读取器是否有行代码>是的,我知道,我尝试过,它没有行,就像我的阅读器无法从数据库中获取任何数据。等等,现在它有行了,我不知道如何重新启动我的visual studio和MySQL服务器。顺便说一句,谢谢你的帮助