multithreading - C# manual lock/unlock

Question

I have a function in C# that can be called multiple times from multiple threads and I want it to be done only once so I thought about this:

class MyClass
{
    bool done = false;
    public void DoSomething()
    {
        lock(this)
            if(!done)
            {
                done = true;
                _DoSomething();
            }
    }
}

The problem is _DoSomething takes a long time and I don't want many threads to wait on it when they can just see that done is true.
Something like this can be a workaround:

class MyClass
{
    bool done = false;
    public void DoSomething()
    {
        bool doIt = false;
        lock(this)
            if(!done)
                doIt = done = true;
        if(doIt)
             _DoSomething();
    }
}

But just doing the locking and unlocking manually will be much better.
How can I manually lock and unlock just like the lock(object) does? I need it to use same interface as lock so that this manual way and lock will block each other (for more complex cases).

Answer 1

The lock keyword is just syntactic sugar for Monitor.Enter and Monitor.Exit:

Monitor.Enter(o);
try
{
    //put your code here
}
finally
{
    Monitor.Exit(o);
}

is the same as

lock(o)
{
    //put your code here
}