Unbuffered QueryAsnyc multi-mapping throws an InvalidOperationException

[mburbea]

using (var conn = new SqlConnection(appToken.ConnectionString))
{
 conn.Open();
 var ret = conn.Query("select a='dog',b=3", (string a, int b) => a + b, splitOn: "b", buffered: false);
 Console.WriteLine(string.Join("", ret));
 ret = conn.QueryAsync("select a='dog',b=3", (string a, int b) => a + b, splitOn: "b", buffered: false).Result;
 Console.WriteLine(string.Join("", ret));
}

The call to Query returns dog3, the QueryAsync throws the following exception:

System.InvalidOperationException was unhandled
 HResult=-2146233079
 Message=Invalid attempt to call FieldCount when reader is closed.
 Source=System.Data
 StackTrace:
 at System.Data.SqlClient.SqlDataReader.get_FieldCount()
 at Dapper.SqlMapper.GetColumnHash(IDataReader reader, Int32 startBound, Int32 length)
 at Dapper.SqlMapper.<MultiMapImpl>d__140`8.MoveNext()
 at System.String.Join(String separator, IEnumerable`1 values)
 at Program.Main(String[] args) in 

The store procedure I am calling takes a while to execute and I'd rather let it be called async before I start streaming data out of the query.

[mburbea]

Figured out the cause.
https://github.com/StackExchange/dapper-dot-net/blob/master/Dapper/SqlMapper.Async.cs#L650
The error is that it returns the enumerable, but the using block closes the connection before enumeration begins.

 using (var reader = await ExecuteReaderWithFlagsFallbackAsync(cmd, wasClosed, CommandBehavior.SequentialAccess | CommandBehavior.SingleResult, command.CancellationToken).ConfigureAwait(false))
 {
 if (!command.Buffered) wasClosed = false; // handing back open reader; rely on command-behavior
 var results = MultiMapImpl<TFirst, TSecond, TThird, TFourth, TFifth, TSixth, TSeventh, TReturn>(null, CommandDefinition.ForCallback(command.Parameters), map, splitOn, reader, identity, true);
 return command.Buffered ? results.ToList() : results;
 }

I'll spin up a PR to fix.

[RodrigoMaf]

Read about and use async/await

Ex.:
using (var conn = new SqlConnection(appToken.ConnectionString))
{
conn.Open();
var ret = conn.Query("select a='dog',b=3", (string a, int b) => a + b, splitOn: "b", buffered: false);
Console.WriteLine(string.Join("", ret));
ret = await conn.QueryAsync("select a='dog',b=3", (string a, int b) => a + b, splitOn: "b", buffered: false).Result;
Console.WriteLine(string.Join("", ret));
}

[0xorial]

Any chance to get this patched?

[kaldas]

Please - Is there any way I can assist in resolving this?

[NickCraver]

There isn't a change to be made here overall - a method using a connection and expected data needs to await the results before closing the connection (at the end of the using). Overall: this is a usage problem, and leaving the reader open won't help...since the connection door will be shut at the end of the using regardless.

I'll cleanup this and the PR, but overall: when reading data, that needs to be completed before severing the connection to the data source.