Question

I’m trying to get a bit more in to design patterns and I was building a quick test to help me further understand singleton pattern. However I came into an error in .net that baffled me, what made it weirder I was not able to replicate the error this just added to my confusion. The code below is not the best code in the world but I was randomly testing things to help me get a deeper understanding.

class Program
{

    static void Main(string[] args)
    {
        Thread t1 = new Thread(new ThreadStart(run1));
        Thread t2 = new Thread(new ThreadStart(run2));

        t1.Priority = ThreadPriority.Lowest;
        t1.Start();
        t2.Priority = ThreadPriority.Lowest;
        t2.Start();

        Console.ReadLine();
    }

    public static void run1()
    {
        Console.WriteLine("im in run1 \n");
        TestSingleton._instance.PopulateCrudItemsProcess();
        Thread.Sleep(1000);
        Console.WriteLine(TestSingleton._instance.GetStrValue("first", 1000));
    }
    public static void run2()
    {
        Console.WriteLine("im in run2 \n");
        TestSingleton._instance.PopulateCrudItemsProcess();
        Console.WriteLine(TestSingleton._instance.GetStrValue("second", 500));

    }

}

sealed class TestSingleton
{
    private TestSingleton() { }

    public static readonly TestSingleton _instance = new TestSingleton();

    public string GetStrValue(string str, int time)
    {
        return str;
    }

    public void PopulateCrudItemsProcess()
    {
        const string proc = "[testdb].[dbo].[tstsproc]";
        string _reportingConnStr = ConfigurationManager.ConnectionStrings["reporting"].ConnectionString;

        using (SqlConnection conn = new SqlConnection(_reportingConnStr))
        {
            using (SqlCommand cmd = new SqlCommand(proc, conn))
            {
                cmd.CommandType = CommandType.StoredProcedure;
                cmd.CommandTimeout = 7200;
                conn.Open();
                cmd.ExecuteNonQuery();
            }
        }
    }
}

Now what happened was the program crashed on cmd.ExecuteNonQuery(); and stated that there was a deadlock on a resource. I wish i could have captured the actual error but like i said I was only able to get the error once. According to MS this code is a thread safe way to create a singleton. So any thoughts on what may have caused the error? any info is greatly appreciated.

Answer 1

You could try using lock keyword.
It allows developers to make scope of codes that must be synch between application threading.
This is to insure that incoming threads will not interrupt the current until it is finished.

So let’s an example:

declare inside your singleton a global

private object _oLock;

rewrite PopulateCrudItemsProcess in this way

    lock(_oLock)
    {
        using (SqlConnection conn = new SqlConnection(_reportingConnStr)) 
        { 
            using (SqlCommand cmd = new SqlCommand(proc, conn)) 
            { 
                cmd.CommandType = CommandType.StoredProcedure; 
                cmd.CommandTimeout = 7200; 
                conn.Open(); 
                cmd.ExecuteNonQuery(); 
            } 
        } 
   }