Composite, State or Strategy? Not again!..

Hi there,

I have a class "Task" that *can* contain many subtasks. Various aspects of
the Tasks behaviour change depending upon whether it contains subtasks or
not.

Currently these varying behaviours are implemented using "if-else"
statements within each method. For example:

public TimeSpan GetDuration()
{
if ( this.subTasks.count > 0 )
{
return sumOfMySubtasksDurations();
}
else
{
return myDuration;
}
}

There is lots of this type of thing going on. I'm at the stage where the
class is 300+ lines of code long and I'm thinking of breaking it down. My
obvious choice is to create two State objects for the Task -
CompositeTaskState and LeafTaskState. The task will assign itself a new
state every time a subtask is added or removed, at runtime. This will create
something like the following:

public TimeSpan GetDuration()
{
return myState.GetDuration();
}

The state itself might be added in the following way:

public void AddSubtask( Task t )
{
subTasks.Add( t );
myState = new CompositeTaskState( this );
}

public void RemoveTask( int i )
{
this.subTasks.RemoveAt( i );
if ( subTasks.Count == 0 )
{
myState = new LeafTaskState();
}
}

You get the idea. I had considered a kind-of-strategy pattern, but this
would mean me creating many strategy patterns for only one single axis of
change - somewhat overkill. Similarly, I would have considered composite,
except this doesn't allow for runtime changes or behaviour? Or does it!?

Any thoughts apprecitated?

Tobes