Building Future<T>


The last post dealt with building the base Future class.  Now we’ll build the child class used to run Func<TResult>‘s. 

The basic implementation is straight forward.  The class will run a delegate typed to Func<TResult> in the override of RunCore.  The trickiest part is how to store the value.  The value is set on one thread and read off of another. 

When a value is read and written on multiple threads there are a couple of options for synchronization between threads.  One of them is to use the volatile keyword for the data.  This forces the CLR to read the value from memory every time and prevents caching issues between threads.  Unfortunately volatile cannot be applied to an unbounded generic. 

To get around this I’ve declared the value to be of type object.  Whenever the value is accessed by the user of Future<T> a cast is applied to the appropriate type.  This incurs boxing overhead but it’s minimal and in the typical case will be limited to one box and unbox per value type. 

In addition Future<T> adds one new method; Wait;  It’s a combination of calling WaitEmpty followed by returning the value. 

In a perfect world WaitEmpty in Future would really be called Wait and be virtual.  Future<T> would override the method and alter the return type to be T.  Unfortunately C#/VB don’t support covariant return types on virtual method overrides so it’s not possible.  Truthfully I don’t know if this is a C#/VB limitation or a CLR one. 

    public class Future<T> : Future
    {
        private Func<T> m_function;
        private volatile object m_value;

        public T Value
        {
            get { return Wait(); }
        }

        public Future(Func<T> function)
        {
            m_function = function;
        }

        public T Wait()
        {
            base.WaitEmpty();
            return (T)m_value;
        }

        protected override void RunCore()
        {
            m_value = m_function();
        }

    }

Next time I’ll go over the implementation of Futures which return no values. 

Comments (4)

  1. Marc Brooks says:

    I would much rather leave the T unconstrained and unboxed and just allocate a (dirt cheap) object to do the locking on…

  2. jaredpar says:

    I considered both approaches.  I agree that boxing is not ideal.  But even though allocating another object is dirt cheap, locking is not.  In particular this would require adding a lock{} block into a property get accessor which seems a bit heavy.

    Boxing/unboxing while unfortunate is also fairly cheap since it’s just another object allocation.  

  3. In addition to Future&lt;T&gt; there is also the concept of Futures that don’t return any values.&#160;

  4. In addition to Future&lt;T&gt; there is also the concept of Futures that don&#39;t return any values