DapperLib
Unbuffered QueryAsnyc multi-mapping throws an InvalidOperationException
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.
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.
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)); }
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.
I agree with the OP that this is a Dapper bug, not a usage problem. Here's the source of MultiMapAsync:
https://github.com/DapperLib/Dapper/blob/b452fb1999c77b08643c7b5c003bd5cd9c793f40/Dapper/SqlMapper.Async.cs#L912-L915
On line 912, a DbDataReader is created using a using declaration. On line 915, an IEnumerable<T> from MultiMapImpl is returned. This is an iterator method that's implemented by a state machine. It still has a reference to the DbDataReader passed to the method, but that DbDataReader will be disposed after the return on line 915 is executed.
Thus, the IEnumerable state machine has a reference to a disposed DbDataReader, and reading it will fail.
The solution might be to transfer ownership of the DbDataReader to MultiMapImpl for non-buffered async reads.
@bgrainger Oh geez, I misread the example missing the .Result up there - agreed, reopening this one.
Bump. Is there a reason @mburbea 's PR can't be merged? Is our only recourse to use the sync version?
