diff --git a/83 Stock Market/csharp/src/Extensions/ImmutableArrayExtensions.cs b/83 Stock Market/csharp/src/Extensions/ImmutableArrayExtensions.cs
new file mode 100644
index 00000000..544c7625
--- /dev/null
+++ b/83 Stock Market/csharp/src/Extensions/ImmutableArrayExtensions.cs	
@@ -0,0 +1,37 @@
+using System;
+using System.Collections.Immutable;
+
+namespace Game.Extensions
+{
+    /// <summary>
+    /// Provides additional methods for the <see cref="ImmutableArray{T}"/> class.
+    /// </summary>
+    public static class ImmutableArrayExtensions
+    {
+        /// <summary>
+        /// Maps each element in an immutable array to a new value.
+        /// </summary>
+        /// <typeparam name="TSource">
+        /// The type of elements in the source array.
+        /// </typeparam>
+        /// <typeparam name="TResult">
+        /// The type of elements in the resulting array.
+        /// </typeparam>
+        /// <param name="source">
+        /// The source array.
+        /// </param>
+        /// <param name="selector">
+        /// Function which receives an element from the source array and its
+        /// index and returns the resulting element.
+        /// </param>
+        public static ImmutableArray<TResult> Map<TSource, TResult>(this ImmutableArray<TSource> source, Func<TSource, int, TResult> selector)
+        {
+            var builder = ImmutableArray.CreateBuilder<TResult>(source.Length);
+
+            for (var i = 0; i < source.Length; ++i)
+                builder.Add(selector(source[i], i));
+
+            return builder.MoveToImmutable();
+        }
+    }
+}
diff --git a/83 Stock Market/csharp/src/Program.cs b/83 Stock Market/csharp/src/Program.cs
index 8d2b2a98..a7ab841a 100644
--- a/83 Stock Market/csharp/src/Program.cs	
+++ b/83 Stock Market/csharp/src/Program.cs	
@@ -9,7 +9,7 @@ namespace Game
         /// <summary>
         /// Defines the set of companies that will be simulated in the game.
         /// </summary>
-        private static ImmutableArray<Company> Companies = ImmutableArray.CreateRange(new[]
+        private readonly static ImmutableArray<Company> Companies = ImmutableArray.CreateRange(new[]
         {
             new Company("INT. BALLISTIC MISSILES",     "IBM", sharePrice:100),
             new Company("RED CROSS OF AMERICA",        "RCA", sharePrice:85 ),
diff --git a/83 Stock Market/csharp/src/StockMarket.cs b/83 Stock Market/csharp/src/StockMarket.cs
index ba2f52ee..36201074 100644
--- a/83 Stock Market/csharp/src/StockMarket.cs	
+++ b/83 Stock Market/csharp/src/StockMarket.cs	
@@ -38,13 +38,13 @@ namespace Game
                 },
                 (parameters, previousDay) => previousDay with
                 {
-                    Companies = ImmutableArray.CreateRange(
-                        previousDay.Companies.Select ((company, index) => AdjustSharePrice(
+                    Companies = previousDay.Companies.Map(
+                        (company, index) => AdjustSharePrice(
                             random,
                             company,
                             parameters.trend,
                             parameters.positiveSpike == index,
-                            parameters.negativeSpike == index)))
+                            parameters.negativeSpike == index))
                 });
         }
 
@@ -106,7 +106,7 @@ namespace Game
         /// The maximum number of days each trend should last.
         /// </param>
         public static IEnumerable<double> Trends(Random random, int minDays, int maxDays) =>
-            random.Integers(minDays, maxDays + 1).SelectMany(days => Enumerable.Repeat(GenerateTrend(random), days));
+            random.Integers(minDays, maxDays + 1).SelectMany(daysInCycle => Enumerable.Repeat(GenerateTrend(random), daysInCycle));
 
         /// <summary>
         /// Generates a random value for the market trend.
@@ -143,7 +143,7 @@ namespace Game
         private static IEnumerable<int?> PriceSpikes(Random random, int companyCount, int minDays, int maxDays) =>
             random.Integers(minDays, maxDays + 1)
                 .SelectMany(
-                    days => Enumerable.Range(0, days),
-                    (days, dayNumber) => dayNumber == 0 ? random.Next(companyCount) : default(int?));
+                    daysInCycle => Enumerable.Range(0, daysInCycle),
+                    (daysInCycle, dayNumber) => dayNumber == 0 ? random.Next(companyCount) : default(int?));
     }
 }