Merge pull request #1970 from Clockwork-Muse/System.Linq.Parallel_CastArgumentNull

Add missing null-argument check to System.Linq.Parallel.Cast().

Author: stephentoub

src/System.Linq.Parallel/src/System/Linq/ParallelEnumerable.cs

@@ -5363,6 +5363,8 @@ public static ParallelQuery<TResult> OfType<TResult>(this ParallelQuery source)
         /// </exception>
         public static ParallelQuery<TResult> Cast<TResult>(this ParallelQuery source)
         {
+            if (source == null) throw new ArgumentNullException("source");
+
             return source.Cast<TResult>();
         }
 

src/System.Linq.Parallel/tests/QueryOperators/CastTests.cs

@@ -118,10 +118,9 @@ public static void Cast_Assignable_InvalidCastException(Labeled<ParallelQuery<in
         }
 
         [Fact]
-        public static void Cast_NullReferenceException()
+        public static void Cast_ArgumentNullException()
         {
-            // Everything else throws ArgumentNullException, but Cast is missing the check that would do so.
-            Assert.Throws<NullReferenceException>(() => ((ParallelQuery<object>)null).Cast<int>());
+            Assert.Throws<ArgumentNullException>(() => ((ParallelQuery<object>)null).Cast<int>());
         }
 
         private class Castable