使用ExecuteReaderAsync时无法将DbDataReader类型隐式转换为MySqlDataReader

Question

I've the following function that allows me to pass in a object and populate that object with the returning data, if any.

I've modified the function so that it can be called asynchronously.

public static async Task<MySqlDataReader> MySQLReturnReader(string sName, List<MySqlParameter> oParameters, Action<MySqlDataReader> fn)
{
    using (MySqlConnection oConn = new MySqlConnection(MasterConn))
    {
        await oConn.OpenAsync();

        using (MySqlCommand oMySqlCommand = new MySqlCommand(sName, oConn))
        {
           oMySqlCommand.CommandType = CommandType.StoredProcedure;

            if (oParameters != null)
            {
                foreach (MySqlParameter oPar in oParameters)
                {
                    oMySqlCommand.Parameters.Add(oPar);
                }
            }

            oMySqlCommand.Connection.Open();

            using (MySqlDataReader oReader = oMySqlCommand.ExecuteReaderAsync())
            {
               fn(oReader);
            }

        }
    }
    return;
}

My class object is something like;

public class MyClass
{
     public int Id {get;set;}
     public string Name {get;set;}
     ...
}

The function can be called like

 List<MyClass> oMyClassList = new List<MyClass>();
 List<MySqlParameter> oParams = new List<MySqlParameter>();

 List<int> Ids = new List<int>(500);
 Ids.Add(1);
 Ids.Add(2);
 ...
 Ids.Add(499);

 foreach(int Id in Ids)
 {
    MySQLReturnReader("SPCheck", oParams, oRes =>
        {
                while (oRes.Read())
                {
                   MyClass oMyClass = new MyClass();                      
                   oMyClass.Id = Convert.ToInt32(oRes["Id"]);                       
                   oMyClass.Name = oRes["Name"].ToString();
                }

                oMyClassList.Add(oMyClass);
        }
        );


 }

The problem is I'm getting the compilation error of 'Cannot implicitly convert type 'System.Threading.Tasks.Task' to 'MySql.Data.MySqlClient.MySqlDataReader'. Where am I going wrong ?

I'm wanting to use ExecuteReaderAsync in this way, as the Stored procedure called is very complex and would prefer to run the requests in parallel.

Answer 1

In your code, you have:

using (MySqlDataReader oReader = oMySqlCommand.ExecuteReaderAsync())

The compiler error Cannot implicitly convert type 'System.Threading.Tasks.Task' to 'MySql.Data.MySqlClient.MySqlDataReader' means that you need to use the await keyword to "unwrap" the task returned by ExecuteReaderAsync :

using (MySqlDataReader oReader = await oMySqlCommand.ExecuteReaderAsync())

Note however that if you're using the official MySql.Data package, this call won't actually execute asynchronously. The Async methods in the MySql.Data connector aren't asynchronous; they block on network I/O and only return when the DB operation is complete. (For a much more detailed discussion, see this question and its top answer .) MySQL bug #70111 reports this problem in the MySQL connector.

To get truly asynchronous DB operations, you'll have to wait until that bug is fixed, or switch to a different connector. I've been developing a new, fully async connector that should be a drop-in replacement for MySql.Data; to try it out, install MySqlConnector from NuGet ; its source is at GitHub .

Answer 2

This most likely indicates that the library you're using doesn't support ExecuteReaderAsync() , so you're just calling the default implementation inherited from DbCommand . This is why it returns the general DbDataReader (instead of the MySQL-specific one). And this also means that your code won't actually be asynchronous, the default version of ExecuteReaderAsync() is just a synchronous wrapper around ExecuteReader() .

So, I think you should directly use the old ExecuteReader() , until your library adds support for ExecuteReaderAsync() .