public sealed class DistinctCountAggregateFunction : AggregateFunction
    {
        public override string DisplayName
        {
            get
            {
                return "Distinct Count";
            }
        }

        protected override AggregateValue CreateAggregate(IAggregateContext context)
        {
            return new DistinctCountAggregate();
        }

        public override string GetStringFormat(Type dataType, string format)
        {
            return "G";
        }

        public override int GetHashCode()
        {
            return 1;
        }

        public override bool Equals(object obj)
        {
            return obj is DistinctCountAggregateFunction;
        }

        public override string ToString()
        {
            return "Distinct Count";
        }

        protected override Cloneable CreateInstanceCore()
        {
            return new DistinctCountAggregateFunction();
        }

        protected override void CloneCore(Cloneable source)
        {
        }
    }

public class DistinctCountAggregate : AggregateValue, IConvertibleAggregateValue<double>
    {
        private HashSet<object> aggregateValues = new HashSet<object>();

        protected override object GetValueOverride()
        {
            return this.aggregateValues.Count;
        }

        protected override void AccumulateOverride(object item)
        {
            this.aggregateValues.Add(item);
        }

        protected override void MergeOverride(AggregateValue childAggregate)
        {
            DistinctCountAggregate distinctCountAggregate = childAggregate as DistinctCountAggregate;
            if (distinctCountAggregate == null)
            {
                return;
            }

            foreach (var aggregateValue in distinctCountAggregate.aggregateValues)
            {
                this.aggregateValues.Add(aggregateValue);
            }
        }

        bool IConvertibleAggregateValue<double>.TryConvertValue(out double value)
        {
            value = Convert.ToDouble(this.GetValueOverride());
            
            return true;
        }
    }

this.provider.AggregateDescriptions.Add(new PropertyAggregateDescription()
{
    PropertyName = "Product",
    AggregateFunction = new DistinctCountAggregateFunction()
});