Inane Ramblings of a .NET Developer - Fast algorithm for continuous mean

public class ContinuousAverage
{
   private int count = 0; 
   private decimal? currentAverage = null ; 

   public decimal Average 
   {
      get 
      {
      if (currentAverage == null) 
         throw new InvalidOperationException ("No numbers have been averaged"); 
   
      return currentAverage.Value; 
      }
   }

   public int Count 
   {
      get { return count; } 
   }

   public void Add(decimal number) 
   {
      count++;

      if (currentAverage == null) 
      {
         currentAverage = number;
      }
      else 
      {
         decimal differenceFromAverage = number - currentAverage.Value; 
         currentAverage += differenceFromAverage / count;
      }
   }
}

[TestFixture]
public class ContinuousAverageTests
{
   [Test] 
   [ExpectedException(typeof(InvalidOperationException ), "No numbers have been averaged")] 
   public void TestAverageOfNoItemsThrowsException( ) 
   {
      MovingAverage movingAverage = new MovingAverage (); 

      decimal value = movingAverage.Average; 
   }

   [Test] 
   public void TestAverageOfOneItemIsItemValue( ) 
   {
      MovingAverage movingAverage = new MovingAverage (); 
      movingAverage.Add( 1.234M );

      Assert.AreEqual( 1.234M, movingAverage.Average ); 
   }

   [Test] 
   public void TestAverageOfSeveralItemsIsStandardArithmeticMean() 
   {
      MovingAverage movingAverage = new MovingAverage (); 

      movingAverage.Add(6);
      movingAverage.Add(2);
      movingAverage.Add(4);
      movingAverage.Add(5);
      movingAverage.Add(6);
      movingAverage.Add(3);
      movingAverage.Add(2);

      Assert.AreEqual(7, movingAverage.Count);
      Assert.AreEqual(4, movingAverage.Average, "(6+2+4+5+6+3+2)/7 = (28/7) = 4"); 
   }
}