Thread safe coding by using lock statement

Question

I am having some doubts while using lock statement to make thread safe, here is my code:

public class Class1
{
    public ClassName _Obj;
    private static object LockObj = new object();

    public ClassName Obj
    {
        get { return _Obj ?? (_Obj = new ClassName()); }
    }

    public void ThreadA()
    {
        //lock (LockObj) --able to handle the thread
        //lock (Obj) --it is also able to handle the thread
        lock (this) // --same for this also
        {
            for (int i = 0; i < 1000; i++)
            {
                Obj.ValueA = i;
                // Processing on Obj..
            }
        }
    }
}

public class ClassName
{
    public int ValueA { get; set; }
}

In this block of code ThreadA can be made safe by using a lock statement. But which is the exact way of passing an argument to the lock statement? The same result can be achieved by passing ClassName object i.e Obj, LockObj object which is static or by this object also. Please make it clear, which is better and why?

Answer 1

From MSDN to explain lock statement:

In general, avoid locking on a public type, or instances beyond your code's control. The common constructs lock (this), lock (typeof (MyType)), and lock ("myLock") violate this guideline:

• lock (this) is a problem if the instance can be accessed publicly.

• lock (typeof (MyType)) is a problem if MyType is publicly accessible.

• lock("myLock") is a problem because any other code in the process using the same string, will share the same lock.

Best practice is to define a private object to lock on, or a private static object variable to protect data common to all instances.