public string[] Make(int totalWidth)
        {
            if(totalWidth % 2 == 0)
                throw new ArgumentException("Total width must be odd");

            var result = new List<string>();
            var centre = totalWidth / 2;
            var increment = 1;
            var currentDistanceFromCentre = 0;
            do
            {
                var letter = (char) (currentDistanceFromCentre + 'A');
                var line =  new string('-', totalWidth).ToArray();   
                line[centre - currentDistanceFromCentre] = letter;
                line[centre + currentDistanceFromCentre] = letter;
                result.Add(new string(line));                      

                // reverse direction when we hit the edge
                if ((2 * currentDistanceFromCentre) + 1 == totalWidth)     
                    increment = -1;

                currentDistanceFromCentre += increment; 
            } while (currentDistanceFromCentre >= 0);                      

            return result.ToArray();
        }
    }

    [TestClass]
    public class DiamondMakerTest
    {
        [TestMethod]
        public void TestSmallDiamond()
        {
            var diamond = new DiamondMaker().Make(5);

            Assert.AreEqual(
@"--A--
-B-B-
C---C
-B-B-
--A--",
      string.Join(Environment.NewLine, diamond));
        }

        [TestMethod]
        public void TestBigDiamond()
        {
            var diamond = new DiamondMaker().Make(9);

            Assert.AreEqual(
@"----A----
---B-B---
--C---C--
-D-----D-
E-------E
-D-----D-
--C---C--
---B-B---
----A----",
      string.Join(Environment.NewLine, diamond));
        }
    
    }