Fix checking equivalence of null subject to dictionary (#933)

Src/FluentAssertions/Equivalency/GenericDictionaryEquivalencyStep.cs

@@ -44,11 +44,19 @@ private static bool PreconditionsAreMet(IEquivalencyValidationContext context, I
             Type expectationType = config.GetExpectationType(context);
 
             return AssertImplementsOnlyOneDictionaryInterface(context.Expectation)
+                   && AssertSubjectIsNotNull(context.Subject)
                    && AssertExpectationIsNotNull(context.Subject, context.Expectation)
                    && AssertIsCompatiblyTypedDictionary(expectationType, context.Subject)
                    && AssertSameLength(context.Subject, expectationType, context.Expectation);
         }
 
+        private static bool AssertSubjectIsNotNull(object subject)
+        {
+            return AssertionScope.Current
+                .ForCondition(!(subject is null))
+                .FailWith("Expected {context:Subject} not to be {0}.", new object[] { null });
+        }
+
         private static bool AssertExpectationIsNotNull(object subject, object expectation)
         {
             return AssertionScope.Current

Tests/Shared.Specs/DictionaryEquivalencySpecs.cs

@@ -377,6 +377,50 @@ public void When_a_read_only_dictionary_does_not_match_the_expectation_it_should
             act.Should().Throw<XunitException>().WithMessage("Expected item[0]*Value3*Value2*");
         }
 
+        [Fact]
+        // #930
+        public void When_a_dictionary_is_compared_to_null_it_should_not_throw_a_NullReferenceException()
+        {
+
+            //-----------------------------------------------------------------------------------------------------------
+            // Arrange
+            //-----------------------------------------------------------------------------------------------------------
+            Dictionary<int, int> subject = null;
+            Dictionary<int, int> expectation = new Dictionary<int, int>();
+
+            //-----------------------------------------------------------------------------------------------------------
+            // Act
+            //-----------------------------------------------------------------------------------------------------------
+            Action act = () => subject.Should().BeEquivalentTo(expectation);
+
+            //-----------------------------------------------------------------------------------------------------------
+            // Assert
+            //-----------------------------------------------------------------------------------------------------------
+            act.Should().Throw<XunitException>();
+        }
+
+        [Fact]
+        // #930
+        public void When_a_null_dictionary_is_compared_to_null_it_should_not_throw()
+        {
+
+            //-----------------------------------------------------------------------------------------------------------
+            // Arrange
+            //-----------------------------------------------------------------------------------------------------------
+            Dictionary<int, int> subject = null;
+            Dictionary<int, int> expectation = null;
+
+            //-----------------------------------------------------------------------------------------------------------
+            // Act
+            //-----------------------------------------------------------------------------------------------------------
+            Action act = () => subject.Should().BeEquivalentTo(expectation);
+
+            //-----------------------------------------------------------------------------------------------------------
+            // Assert
+            //-----------------------------------------------------------------------------------------------------------
+            act.Should().NotThrow();
+        }
+
         [Fact]
         public void
             When_a_dictionary_property_is_detected_it_should_ignore_the_order_of_the_pairs