C# c mysql连接器varchar输出参数

我有一个mysql v5.6.25在windows 8.1存储过程上运行，它执行以下操作：

DELIMITER $$
CREATE PROCEDURE `usp_GetData`(IN param1 int, IN param2 datetime, IN param3 datetime, OUT outparam varchar(2054))
BEGIN
    SELECT things FROM stuff WHERE a = param1 AND b = param2 AND c = param3;
    SET outparam = (SELECT GROUP_CONCAT(otherthings) FROM stuff WHERE a = param1 AND b = param2 AND c = param3);

END;

MySql.Data.MySqlClient.MySqlCommand command = new MySqlCommand();
command.CommandText = StoredProcName;
command.CommandType = System.Data.CommandType.StoredProcedure;
MySql.Data.MySqlClient.MySqlParameter outparamobj = new MySqlParameter("outparam",MySqlDbType.VarChar);
outparamobj.Direction = System.Data.ParameterDirection.Output;

command.Parameters.Add(outparamobj);
command.Parameters.AddWithValue("param1", 1);   
command.Parameters.AddWithValue("param2", startdate);
command.Parameters.AddWithValue("param3", enddate);

command.Connection = DBConn; //object set elsewhere but is an open connection.
using (MySql.Data.MySqlClient.MySqlDataReader reader = command.ExecuteReader())
{
    while (reader.Read())
    {
        //populate data object  
    }
    //to make out parameter work - ensure the reader is closed.
    if(!reader.IsClosed) reader.Close();
    string outparamvalue = command.Parameters["outparam"].Value.ToString();
}

我通过mysqlconnector使用以下命令从c调用它：

DELIMITER $$
CREATE PROCEDURE `usp_GetData`(IN param1 int, IN param2 datetime, IN param3 datetime, OUT outparam varchar(2054))
BEGIN
    SELECT things FROM stuff WHERE a = param1 AND b = param2 AND c = param3;
    SET outparam = (SELECT GROUP_CONCAT(otherthings) FROM stuff WHERE a = param1 AND b = param2 AND c = param3);

END;

MySql.Data.MySqlClient.MySqlCommand command = new MySqlCommand();
command.CommandText = StoredProcName;
command.CommandType = System.Data.CommandType.StoredProcedure;
MySql.Data.MySqlClient.MySqlParameter outparamobj = new MySqlParameter("outparam",MySqlDbType.VarChar);
outparamobj.Direction = System.Data.ParameterDirection.Output;

command.Parameters.Add(outparamobj);
command.Parameters.AddWithValue("param1", 1);   
command.Parameters.AddWithValue("param2", startdate);
command.Parameters.AddWithValue("param3", enddate);

command.Connection = DBConn; //object set elsewhere but is an open connection.
using (MySql.Data.MySqlClient.MySqlDataReader reader = command.ExecuteReader())
{
    while (reader.Read())
    {
        //populate data object  
    }
    //to make out parameter work - ensure the reader is closed.
    if(!reader.IsClosed) reader.Close();
    string outparamvalue = command.Parameters["outparam"].Value.ToString();
}

参数的值在mysqlparameter对象中保持为dbnull，即使在MySQL工作台中调用时返回值。如果我将out参数类型更改为int并返回项目计数，则将成功返回，因此我怀疑我没有正确定义参数的类型，但没有成功

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非常感谢任何帮助

也许您应该按照过程中定义的顺序，最后添加输出参数，而不是第一个。感谢您的关注。我一开始试过了，但没用-我在处理排序问题，因为我知道必须先添加一些返回参数。@Neil outparamname的值是多少？这行中的参数不应该是outparam MySql.Data.MySqlClient.MySqlParameter outparamobj=新的MySqlParameteroutparamname，MySqlDbType.VarChar；这不应该是MySql.Data.MySqlClient.MySqlParameter outparamobj=new MySqlParameter outparam，MySqlDbType.VarChar；MySql在Sql Server中声明参数时也支持@symbol。抱歉，我本可以说得更清楚。outparamname是一个值为outparam的字符串变量。将编辑问题。我不确定Mysql是否支持“@”，但对于int类型的参数，它不需要它，对于varchar参数，它不能使用“@”或不使用“@”。