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Unbuffered QueryAsnyc multi-mapping throws an InvalidOperationException #596
Description
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.
Activity
mburbea commentedon Sep 1, 2016
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.
I'll spin up a PR to fix.
Allow multi-map async queries to be unbuffered
RodrigoMaf commentedon Jan 18, 2018
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 commentedon Oct 5, 2020
Any chance to get this patched?
kaldas commentedon Feb 3, 2021
Please - Is there any way I can assist in resolving this?
NickCraver commentedon May 8, 2021
There isn't a change to be made here overall - a method using a connection and expected data needs to
awaitthe results before closing the connection (at the end of theusing). 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 theusingregardless.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.
bgrainger commentedon May 8, 2021
I agree with the OP that this is a Dapper bug, not a usage problem. Here's the source of
MultiMapAsync:Dapper/Dapper/SqlMapper.Async.cs
Lines 912 to 915 in b452fb1
On line 912, a
DbDataReaderis created using ausingdeclaration. On line 915, anIEnumerable<T>fromMultiMapImplis returned. This is an iterator method that's implemented by a state machine. It still has a reference to theDbDataReaderpassed to the method, but thatDbDataReaderwill be disposed after thereturnon 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
DbDataReadertoMultiMapImplfor non-buffered async reads.NickCraver commentedon May 8, 2021
@bgrainger Oh geez, I misread the example missing the
.Resultup there - agreed, reopening this one.Dean-NC commentedon May 15, 2026
I'd also like to have unbuffered async multi-mapping. Since the reader is disposed inside MultiMapAsync(), you'll get an exception if you use the enumerable unbuffered.