Merge pull request aalhour#134 from RyanGrange/issue129

Issue 129: Return primes list as read-only.

Author: aalhour

DataStructures/Common/PrimesList.cs

@@ -184,11 +184,11 @@ public int GetPreviousPrime(int number)
         }
 
         /// <summary>
-        /// Returns the list of primes
+        /// Returns the read-only IList of primes
         /// </summary>
-        public List<int> GetAll
+        public IList<int> GetAll
         {
-            get { return _primes; }
+            get { return _primes.AsReadOnly(); }
         }
 
         /// <summary>

UnitTest/DataStructuresTests/PrimeListTest.cs

@@ -1,4 +1,5 @@
-using DataStructures.Common;
+using System;
+using DataStructures.Common;
 using Xunit;
 
 namespace UnitTest.DataStructuresTests
@@ -11,5 +12,13 @@ public void DoTest()
             var instance = PrimesList.Instance;
             Assert.Equal(10000, instance.Count);
         }
+
+        [Fact]
+        public void PrimesListIsReadOnly()
+        {
+            var instance = PrimesList.Instance;
+            NotSupportedException ex = Assert.Throws<NotSupportedException>(() => instance.GetAll[0] = -1);
+            Assert.Equal("Collection is read-only.", ex.Message);
+        }
     }
 }