Sharing data between threads of different methods

Question

I am curious about the best way to share data between threads that aren't based on the same method.

For example, if I want to share the same string(by reference) between 2 threads:

Main()
{
string shareme;
Start New Thread based on A()
Start New Thread based on B()

shareme = "Updated in main";

}

A()
{
shareme = "Updated in A";
}

B()
{
shareme = "Updated in B";
}

Thanks

Vulpes · Answer

The usual method is to use locks (as you would if it were the same method) to achieve predictable behavior though you might need to use other techniques as well. For example if we make your code into an executable program and run it multiple times: using System; using System.Threading; class Program { string shareme; static Program p = new Program(); static void Main() { Program p = new Program(); Thread t1 = new Thread(p.A); Thread t2 = new Thread(p.B); t1.Start(); t2.Start(); p.shareme = Updated in main; Console.WriteLine(p.shareme); Console.ReadKey(); } void A() { p.shareme = Updated in A; Console.WriteLine(p.shareme); } void B() { p.shareme = Updated in B; Console.WriteLine(p.shareme); } } The order of the output is completely unpredictable. It might be : Updated in main Updated in A Updated in B bit it could be other permutations as well. Suppose we wanted it to always be: Updated in A Updated in B Updated in main then we could achieve that by adding a lock in A() and then 'joining' both threads in the main thread so that the latter's Console.WriteLine always executes last: using System; using System.Threading; class Program { static object padLock = new object(); string shareme; static Program p = new Program(); static void Main() { Program p = new Program(); Thread t1 = new Thread(p.A); Thread t2 = new Thread(p.B); t1.Start(); t1.Join(); t2.Start(); t2.Join(); p.shareme = Updated in main; Console.WriteLine(p.shareme); Console.ReadKey(); } void A() { lock(padLock) { p.shareme = Updated in A; Console.WriteLine(p.shareme); } } void B() { p.shareme = Updated in B; Console.WriteLine(p.shareme); } } The lock ensures that the B thread doesn't have a chance to update shareme before thread A can print it to the console.

Vulpes · Answer

Well if you call t.Join() then it blocks the calling thread until 't' completes. So in the earlier example we were blocking the main thread until both the A and B threads had completed. You can do this for as many threads as you like but you don't have to do it for all the threads that have been spawned. So the main thread could have started 8 other threads and joined to 4 of them. The remaining 4 threads would then (if they hadn't finished already) have run concurrently with the main thread after it had 'woken up' from the joins with the other 4 threads.