问题

I am extracting content of the Files in SQL File Table. The following code works if I do not use Parallel.

I am getting the following exception, when reading sql file stream simultaneously (Parallel).

The process cannot access the file specified because it has been opened in another transaction.

TL;DR:

When reading a file from FileTable (using GET_FILESTREAM_TRANSACTION_CONTEXT) in a Parallel.ForEach I get the above exception.

Sample Code for you to try out:

https://gist.github.com/NerdPad/6d9b399f2f5f5e5c6519

Longer Version:

Fetch Attachments, and extract content:

var documents = new List<ExtractedContent>();
using (var ts = new TransactionScope(TransactionScopeAsyncFlowOption.Enabled))
{
    var attachments = await dao.GetAttachmentsAsync();

    // Extract the content simultaneously
    // documents = attachments.ToDbDocuments().ToList(); // This works
    Parallel.ForEach(attachments, a => documents.Add(a.ToDbDocument())); // this doesn't

    ts.Complete();
}

DAO Read File Table:

public async Task<IEnumerable<SearchAttachment>> GetAttachmentsAsync()
{
    try
    {
        var commandStr = "....";

        IEnumerable<SearchAttachment> attachments = null;
        using (var connection = new SqlConnection(this.DatabaseContext.Database.Connection.ConnectionString))
        using (var command = new SqlCommand(commandStr, connection))
        {
            connection.Open();

            using (var reader = await command.ExecuteReaderAsync())
            {
                attachments = reader.ToSearchAttachments().ToList();
            }
        }

        return attachments;
    }
    catch (System.Exception)
    {
        throw;
    }
}

Create objects for each file: The object contains a reference to the GET_FILESTREAM_TRANSACTION_CONTEXT

public static IEnumerable<SearchAttachment> ToSearchAttachments(this SqlDataReader reader)
{
    if (!reader.HasRows)
    {
        yield break;
    }

    // Convert each row to SearchAttachment
    while (reader.Read())
    {
        yield return new SearchAttachment
        {
            ...
            ...
            UNCPath = reader.To<string>(Constants.UNCPath),
            ContentStream = reader.To<byte[]>(Constants.Stream) // GET_FILESTREAM_TRANSACTION_CONTEXT() 
            ...
            ...
        };
    }
}

Read the file using SqlFileStream: Exception is thrown here

public static ExtractedContent ToDbDocument(this SearchAttachment attachment)
{
    // Read the file
    // Exception is thrown here
    using (var stream = new SqlFileStream(attachment.UNCPath, attachment.ContentStream, FileAccess.Read, FileOptions.SequentialScan, 4096))
    {
        ...
        // extract content from the file
    }

    ....
}

Update 1:

According to this article it seems like it could be an Isolation level issue. Has anyone ever faced similar issue?

回答1:

The transaction does not flow in to the Parallel.ForEach, you must manually bring the transaction in.

//Switched to a thread safe collection.
var documents = new ConcurrentQueue<ExtractedContent>();
using (var ts = new TransactionScope(TransactionScopeAsyncFlowOption.Enabled))
{
    var attachments = await dao.GetAttachmentsAsync();
    //Grab a reference to the current transaction.
    var transaction = Transaction.Current;
    Parallel.ForEach(attachments, a =>
    {
        //Spawn a dependant clone of the transaction
        using (var depTs = transaction.DependentClone(DependentCloneOption.RollbackIfNotComplete))
        {
            documents.Enqueue(a.ToDbDocument());
            depTs.Complete();
        }
    });

    ts.Complete();
}

I also switched from List<ExtractedContent> to ConcurrentQueue<ExtractedContent> because you are not allowed call .Add( on a list from multiple threads at the same time.

来源：https://stackoverflow.com/questions/30177583/threading-and-sqlfilestream-the-process-cannot-access-the-file-specified-becaus

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