SqlMapper.GridReader.ReadUnbufferedAsync possibly hiding exceptions
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Description
We're currently facing issues with querying multiple results and reading each of them unbuffered.
After investigating the Dapper source code I think we may have some exception that is being hidden by Dapper. Our stack trace reveals an exception being thrown in OnAfterGridAsync which is being called in a finally block. I understand that this block is necessary.
async Task Main()
{
await foreach (var record in ReadUnbufferedAsync())
{
// ...
}
}
private async Task OnAfterGridAsync()
{
await Task.Yield();
throw new InvalidOperationException("OnAfterGridAsync");
}
private async Task<bool> ReadAsync()
{
await Task.Yield();
throw new InvalidOperationException("ReadAsync"); // this exception will be hidden by the other
}
private string ConvertTo()
{
return "Record";
}
public async IAsyncEnumerable<string> ReadUnbufferedAsync()
{
try
{
while (await ReadAsync())
{
yield return ConvertTo();
}
}
finally
{
await OnAfterGridAsync();
}
}
However, wouldn't it be possible to apply refactoring to properly propagate exceptions to the call-site?
async Task Main()
{
await foreach (var record in ReadUnbufferedAsync())
{
// ...
}
}
private async Task OnAfterGridAsync()
{
await Task.Yield();
throw new InvalidOperationException("OnAfterGridAsync");
}
private async Task<bool> ReadAsync()
{
await Task.Yield();
throw new InvalidOperationException("ReadAsync");
}
private string ConvertTo()
{
return "Record";
}
public async IAsyncEnumerable<string> ReadUnbufferedAsync()
{
ExceptionDispatchInfo? exceptionCaught = null;
try
{
while (true)
{
string record;
try
{
var hasNext = await ReadAsync();
if (!hasNext)
{
break;
}
record = ConvertTo();
}
catch (Exception ex)
{
exceptionCaught = ExceptionDispatchInfo.Capture(ex);
break;
}
yield return record;
}
}
finally
{
try
{
await OnAfterGridAsync();
}
catch
{
if (exceptionCaught == null)
{
throw;
} // else swallow this exception and propagate the other
}
exceptionCaught?.Throw();
}
}
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First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start in Dapper/SqlMapper.GridReader.Async.cs at the linked ReadUnbufferedAsync implementation, focusing on how OnAfterGridAsync is called during iterator cleanup. Reproduce the provided multiple-exception scenario and determine how the call site should observe the original read failure; done means exceptions are propagated without being unexpectedly hidden.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- csharp
- Domain
- database
- Issue type
- Bug
- Difficulty
- 3/5
- Estimated time
- 1-2 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 42/100