CODEWITHSUNDEEP
c#multithreading
John
John

Reputation: 97

Multiple thread for same method

I am trying to get 100000 string output and trying to achieve with multiple threads but when checking final result string, it only has 10000 line.

Here => 

  string result = "";

   private void Testing()
            {  
 var threadA = new Thread(() => { result += A()+Environment.NewLine; });
                var threadB = new Thread(() => { result += A() + Environment.NewLine; });
                var threadC = new Thread(() => { result += A() + Environment.NewLine; });
                var threadD = new Thread(() => { result += A() + Environment.NewLine; });
                var threadE = new Thread(() => { result += A() + Environment.NewLine; });
                var threadF = new Thread(() => { result += A() + Environment.NewLine; });
                var threadG = new Thread(() => { result += A() + Environment.NewLine; });
                var threadH = new Thread(() => { result += A() + Environment.NewLine; });
                var threadI = new Thread(() => { result += A() + Environment.NewLine; });
                var threadJ = new Thread(() => { result += A()+Environment.NewLine; });

                threadA.Start();
                threadB.Start();
                threadC.Start();
                threadD.Start();
                threadE.Start();
                threadF.Start();
                threadG.Start();
                threadH.Start();
                threadI.Start();
                threadJ.Start();

                threadA.Join();
                threadB.Join();
                threadC.Join();
                threadD.Join();
                threadE.Join();
                threadF.Join();
                threadG.Join();
                threadH.Join();
                threadI.Join();
                threadJ.Join();
    }

private string A()
        {
            for (int i = 0; i <= 10000; i++)
            {
                result += "select * from testing" + Environment.NewLine;
            }
            return result;
        }

But i dont get 100000,I just get 10000.Please let me known why?

Upvotes: 0

Views: 70

Answers (2)

TheGeneral
TheGeneral

Reputation: 81573

Another way you can do this is to forget about creating a Thread it has a lot of overhead and there are many better solutions. Why not just use a Parallel.For. It uses the threadpool you can set how much parallelism you like.

Also if you you are dealing with threads you need to know how to write thread safe code, there are many sorts of locking mechanisms, or there are structures built with thread safety in mind, Thread-Safe Collections . If ordering doesn't matter you could easily use ConcurrentBag<T>

Which can shorten your code to

static ConcurrentBag<string> results  = new  ConcurrentBag<string>();

...

private static void myTest(int count )
{
   for (var i = 0; i < 1000; i++)
   {
      results.Add("select * from testing " + i * count);
   }
}

Usage

Parallel.For(0, 10, myTest);
var result = string.Join(Environment.NewLine, results);

Anyway, this wasn't intended to be a panacea to your problems or the worlds best line writing threaded masterpiece, its just to show you there is lots of resources for threading and many ways to do what you want.

Upvotes: 2

ProgrammingLlama
ProgrammingLlama

Reputation: 38850

As I've explained in the comments, A() is not thread-safe.

If you visualise result += value; as result = result+ value;, you can see that between a single thread getting the result, and writing it back, it's possible for another thread to get the (now) old value.

You should build each thread's contribution in a local variable (I've changed this to StringBuilder since it's more efficient than string concatenation) and then synchronise the context, and update the result object:

private readonly object _resultLock = new object();    
private void A()
{
    var lines = new StringBuilder();
    for (int i = 0; i <= 10000; i++)
    {
        lines.AppendLine("select * from testing");
    }
    lock (_resultLock)
    {
        result += lines.ToString();
    }
}

Since you already have a variable called "result" in the class scope, I've changed A() to a void.

It's best to lock as little as possible, since threads will have to wait to acquire the lock. We use _resultLock so that we know that the lock is for. You can read more about lock in the docs and on this question.

You might also want to look into tasks: docs, Task vs Thread question.

Upvotes: 0

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