Question

I came across some code like this.

public class ConnectionUtility
{
    private static SqlConnection con;

    public static SqlConnection GimmeConnection()
    {
        if(con==null)
            con = new SqlConnection();
        return con;
    }
}

This is in an ASP.NET web application. Can one expect there to be race conditions where one request/page tries to open/close execute things on the connection and other request tries to do those things as well?

Answer 1

Two race conditions, one potentially harmless, one dreadful. There’s also a logical error.

The potentially harmless one is in the constructor logic of:

if(con==null)
  con = new Thing();

This can be harmless if you want to use a single object as an optimisation, but having a period where more than one is used is merely sub-optimal rather than wrong (not every race is the end of the world). It depends on just what Thing is and how it’s used.

The disastrous race is:

return con;

Because in this case the type is SqlConnection, which is not thread-safe, so every single use of the property is a race that is courting disaster.

The logical error is in having a singleton cache an object which is light to produce and which handles its own thread-safe pooling of the heavy part. Because of this pooling, you shouldn’t hold on to an SqlConnection any longer than absolutely necessary. Indeed, if you’ve a gap between one use and another (a bad smell in itself though), you should close it and re-open it again. This makes the thread-safe pooling that SqlConnection provides for you, work at it most optimal between uses.