How to use an AppDomain to limit a static class' scope for thread-safe use?

Question

I have been bitten by a poorly architected solution. It is not thread safe!

I have several shared classes and members in the solution, and during development all was cool...
BizTalk has sunk my battle ship.

We are using a custom BizTalk Adapter to call my assemblies. The Adapter is calling my code and running things in parallel, so I assume it is using multiple threads all under the same AppDomain.

What I would like to do is make my code run under its own AppDomain so the shared problems I have will not muck with each other.

I have a very simple class that the BizTalk adapter is instantiating then running a Process() method.

I would like to create a new AppDomain inside my Process() method, so each time BizTalk spins another thread, it will have its own version of the static classes and methods.

BizTalkAdapter Code:

  // this is inside the BizTalkAdapter and it is calling the Loader class //
  private void SendMessage(IBaseMessage message, TransactionalTransmitProperties properties)
    {

        Stream strm = message.BodyPart.GetOriginalDataStream();
        string connectionString = properties.ConnectionString;
        string msgFileName = message.Context.Read("ReceivedFileName", "http://schemas.microsoft.com/BizTalk/2003/file-properties") as string;


        Loader loader = new Loader(strm, msgFileName, connectionString);
        loader.Process();

        EventLog.WriteEntry("Loader", "Successfully processed: " + msgFileName);

    }

This is the class BizTalk Calls:

public class Loader
{

    private string connectionString;
    private string fileName;
    private Stream stream;
    private DataFile dataFile;

    public Loader(Stream stream, string fileName, string connectionString)
    {
        this.connectionString = connectionString;
        this.fileName = fileName;
        this.stream = stream;
    }  

    public void Process()
    {

        //*****  Create AppDomain HERE *****
        // run following code entirely under that domain
        dataFile = new DataFile(aredStream, fileName, connectionString);
        dataFile.ParseFile();
        dataFile.Save();
        // get rid of the AppDomain here...

    }

}

FYI: The Loader class is in a seperate DLL from the dataFile class.

Any help would be appreciated. I will continue to working on making the code Thread-Safe, but I feel like this could be the "simple" answer.

If anyone has any other thought, please throw in.

Thank you,
Keith

Just for completeness.

I did find that if I marked the send adapter as "Ordered Delivery" in the "Transport Advanced Options" dialog I was able to avoid the multi-thread issues I was having.

I figure this is another possible answer to my problem, but not necessarily to the question.

Answer 1

Using app domains you could do something like this:

public class Loader
{

    private string connectionString;
    private string fileName;
    private Stream stream;
    private DataFile dataFile;

    public Loader(Stream stream, string fileName, string connectionString)
    {
        this.connectionString = connectionString;
        this.fileName = fileName;
        this.stream = stream;
    }  

    public void Process()
    {
        //*****  Create AppDomain HERE *****
        string threadID = Thread.CurrentThread.ManagedThreadId.ToString();
        AppDomain appDomain = AppDomain.CreateDomain(threadID);

        DataFile dataFile = 
            (DataFile) appDomain.CreateInstanceAndUnwrap(
                        "<DataFile AssemblyName>", 
                        "DataFile", 
                        true, 
                        BindingFlags.Default,
                        null,
                        new object[] 
                        { 
                            aredstream, 
                            filename, 
                            connectionString 
                        },
                        null,
                        null,
                        null);
        dataFile.ParseFile();
        dataFile.Save();

        appDomain.Unload(threadID);       
    }
}

Answer 2

Which bit, exactly, is being a pain in terms of thread safety? I can't see any static state nor singletons - and there seems to be appropriate "new" objects... am I being blind?

So what is the symptom you are seeing...

An AppDomain answer will be (relatively) slow. As part of a middleware-backed system this might be OK (i.e. the "relatively" is in the same ball-park).

If you do have some static state somewhere, another option that sometimes works is [ThreadStatic] - which the runtime interprets as "this static field is unique per thread". You need to be careful with initialization, though - the static constructor on thread A might assign a field, but then thread B would see a null/0/etc.

Answer 3

Why not just put a lock around the code you want to execute sequentially? It will be a bottleneck, but it should work in a multithreaded environment.

public class Loader
{
    private static object SyncRoot = new object();
    private string connectionString;
    private string fileName;
    private Stream stream;
    private DataFile dataFile;

    public Loader(Stream stream, string fileName, string connectionString)
    {
        this.connectionString = connectionString;
        this.fileName = fileName;
        this.stream = stream;
    }  

    public void Process()
    {

        lock(SyncRoot) {
            dataFile = new DataFile(aredStream, fileName, connectionString);
            dataFile.ParseFile();
           dataFile.Save();
        }

    }

}

Answer 4

If you have shared statics that are conflicting with each other, then you might want to try adding [ThreadStatic] attribute to them. This will make them local to each thread. That may solve your problem in the short term. A correct solution would be to simply rearchitect your stuff to be thread-safe.

Answer 5

Just for completeness.

I did find that if I marked the send adapter as "Ordered Delivery" in the "Transport Advanced Options" dialog I was able to avoid the multi-thread issues I was having.

I figure this is another possible answer to my problem, but not necessarily to the question.

Answer 6

Creating and tearing down an appdomain for each call - I take it you're not worried about performance on this one?

Ideally you should change the called code to be threadsafe.