Can One Thread Communicate with Another?

Question

Is there a way in C# to send a message to another thread based on the thread's thread id or name?

Basically for a project in school, my professor wants us to do a producer/consumer deal, but passing objects serialized to a string(such as xml) from producer to consumer. Once a string is pulled from a buffer in the consumer thread, each of those strings is decoded(including the threadid) and processed and the original producer is notified via callback. So how do I send an event to the original producer thread with just the thread id?

Answer 1

You can write a class which has a Dictionary< string, thread > member containing all your threads. When you create a thread add it to the dictionary so you can return it by name (key) later from anywhere in the class. This way you can also share resources among your threads, but be sure to lock any shared resources to prevent concurrency issues.

Answer 2

Imagine you run a company, and you could hire as many employees as you liked, but each employee was really single-minded, so you could only give them one order ever. You couldn't get much done with them, right? So if you were a smart manager, what you'd do is say "Your order is 'wait by your inbox until you get a letter telling you what to do, do the work, and then repeat'". Then you could put work items into the worker's inboxes as you needed work done.

The problem then is what happens if you give an employee a long-running, low priority task (let's say, "drive to Topeka to pick up peanut butter for the company picnic"). The employee will happily go off and do that. But then the building catches fire, and you need to know that if you issue the order "grab the fire extinguisher and put the fire out!" someone is going to do that quickly. You can solve that problem by having multiple employees share a single inbox- that way, there is a higher probability that someone will be ready to execute the order to douse the flames, and not be off driving through Kansas.

Guess what? Threads are those difficult employees.

You don't "pass messages to a thread". What you can do is set up a thread or group of threads to observe a common, shared data structure such as a blocking queue (BlockingCollection in .NET, for example), and then put messages (like your strings) into that queue for processing by the consumer threads (which need to listen on the queue for work).

For bidirectional communication, you would need two queues (one for the message, and one for the response). The reason is that your "main" thread is also a bad employee- it only can process responses one at a time, and while it is processing a response from one worker, another worker might come back with another response. You'd want to build a request/response coordination protocol so that the original requestor knows which request a response is associated with- usually requests have an ID, and responses reference the request's ID so that the original requestor knows which request each response is for.

Finally you need proper thread synchronization (locking) on the queues if that isn't built in to the Producer/Consumer queue that you are working with. Imagine if you were putting a message into a worker's inbox, and that worker was so eager to read the message that he grabbed it from your hand and tore it in half. What you need is the ability to prevent more than one thread from accessing the queue at a time.

Answer 3

When using threads you do not try to send messages between them. Threads can use a shared memory to synchronize themselves - This is called synchronized objects. In order to manage threads for a consumer/producer system you can use a queue (a data structure) and not a message system. (see example here: C# producer/consumer).

Another possible solution (which I would not recommend) is : You can use GetThreadId to return the ID of a given native thread. Then all you need to find is the thread handle and pass it to that function. GetCurrentThreadId returns the ID of the current thread. There you can access it's name property.

Answer 4

A message is simply a method call, and to make a method call you first need an instance object which expose some methods to be called, thus sending a message to a thread means finding active object which lives in that thread and calling it's specific method.

Finding each thread's main worker's object could be handled through the threads coordinator, so if an object in a specific thread wants to send a message to another object (in other thread), it would first send it's request to threads coordinator and the coordinator sends the message/request to it's destination.