Static Thread Safety
time to read 12 min | 2240 words

My last post about static variables caused a few comments, so I think that it needs more clarifications. The first thing is to understand what I am talking about when I'm thinking about thread safety.

  • Thread safety issue when you initialize something (which I actually run into today :-) ) where two threads try to access a lazy resource and end up initializing it twice. This is relatively easy to solve using static constructor or static initializators. They are guaranteed to run before somebody access this field for the first time, and they are guaranteed to run once and only once.
    public static readonly IDbConnection Connection = CreateAndOpenConnection(); 

    This is not the kind of thread safety I am talking about here.
  • Thread safety when you use a variable from multiply threads. This is usually what worries me. It worries me because I often work in multiply threads, and because I usually work in the web, where thread affinaty in not something I can rely on. Check out the sample code below.

First, we define the database class:

public class DataBase

{

       public static SqlConnection Connection = CreateAndOpenConnection();

 

       private static SqlConnection CreateAndOpenConnection()

       {

              SqlConnection sqlConnection = new SqlConnection("Data Source=localhost;Initial Catalog=test;Integrated Security=True");

              sqlConnection.Open();

              return sqlConnection;

       }

}

Notice that the SqlConnection variable is public static, and in its documentation there is a comment saying that public static variables of this type are safe for multi threading. The documentation is even current in this instance. It is just talking about access the variable itself, not working with it.

Update: There is something wrong in my reading comprehension. Sergey posted a comment about the above sentence. I read it wrong. It is not public static memebers of this type that I define. It is public static members that this type has. Damn! Sorry about the mistake.

Here is a simple method that works with this object. Notice that there is a slight delay to simulate heavier load:

private static void GetData()

{

       using (IDbCommand command = DataBase.Connection.CreateCommand())

       {

              command.CommandText = "SELECT * FROM Persons";

              using(IDataReader reader = command.ExecuteReader())

              {

                     Thread.Sleep(500);

                     while(reader.Read())

                     {

                           Console.WriteLine(reader.GetValue(0));

                     }

              }

       }

}

Pretty standard stuff so far, isn't it? Now, let us look at our Main method:

static void Main(string[] args)

{

       new Thread(GetData).Start();

       new Thread(GetData).Start();

       Console.ReadKey();

}

We are accessing the GetData() method from two different threads, and both threads end up using the same instance of SqlConnection.

Can you guess what the result will be?

In my tests, it consistently throws one of these exceptions:

  • Invalid attempt to Read when reader is closed.
  • There is already an open DataReader associated with this Command which must be closed first.

We have got ourself a threading problem...

Clarification: The issue that I am raising here is a basic one, I know. I'm trying to reach some place and I want to go all the way, so I wouldn't lose anything along the way. It should be clearer later. (Oh, and the static connection property is evil. It is just the simplest example I could think about).

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