Improve performance of AllBeEquivalentTo

Src/FluentAssertions/Collections/GenericCollectionAssertions.cs

@@ -331,12 +331,22 @@ private bool IsValidProperty<TSelector>(Expression<Func<T, TSelector>> propertyE
         /// Zero or more objects to format using the placeholders in <see cref="because" />.
         /// </param>
         public void AllBeEquivalentTo<TExpectation>(TExpectation expectation,
-            Func<EquivalencyAssertionOptions<TExpectation>, EquivalencyAssertionOptions<TExpectation>> config, string because = "",
+            Func<EquivalencyAssertionOptions<TExpectation>, EquivalencyAssertionOptions<TExpectation>> config,
+            string because = "",
             params object[] becauseArgs)
         {
             TExpectation[] repeatedExpectation = RepeatAsManyAs(expectation, Subject).ToArray();
 
-            BeEquivalentTo(repeatedExpectation, config, because, becauseArgs);
+            // Because we have just manually created the collection based on single element
+            // we are sure that we can force string ordering, because ordering does not matter in terms
+            // of correctness. On the other hand we do not want to change ordering rules for nested objects
+            // in case user needs to use them. Strict ordering improves algorithmic complexity
+            // from O(n^2) to O(n). For bigger tables it is necessary in order to achieve acceptable
+            // execution times.
+            Func<EquivalencyAssertionOptions<TExpectation>, EquivalencyAssertionOptions<TExpectation>> forceStringOrderingConfig =
+                x => config(x).WithStrictOrderingFor(s => s.SelectedMemberPath == "");
+
+            BeEquivalentTo(repeatedExpectation, forceStringOrderingConfig, because, becauseArgs);
         }
 
         private static IEnumerable<TExpectation> RepeatAsManyAs<TExpectation>(TExpectation value, IEnumerable<T> enumerable)

Tests/Shared.Specs/CollectionEquivalencySpecs.cs

@@ -889,7 +889,7 @@ public void When_all_subject_items_are_equivalent_to_expectation_object_it_shoul
         }
 
         [Fact]
-        public void When_all_subject_items_are_not_equivalent_to_expectation_object_it_should_throw()
+        public void When_some_subject_items_are_not_equivalent_to_expectation_object_it_should_throw()
         {
             //-----------------------------------------------------------------------------------------------------------
             // Arrange
@@ -900,14 +900,45 @@ public void When_all_subject_items_are_not_equivalent_to_expectation_object_it_s
             // Act
             //-----------------------------------------------------------------------------------------------------------
             Action action = () => subject.Should().AllBeEquivalentTo(1);
-
+            
             ////-----------------------------------------------------------------------------------------------------------
             //// Assert
             ////-----------------------------------------------------------------------------------------------------------
             action.Should().Throw<XunitException>().WithMessage(
                 "Expected item[1] to be 1, but found 2.*Expected item[2] to be 1, but found 3*");
         }
 
+        [Fact]
+        public void When_some_subject_items_are_not_equivalent_to_expectation_for_huge_table_execution_time_should_still_be_short()
+        {
+            //-----------------------------------------------------------------------------------------------------------
+            // Arrange
+            //-----------------------------------------------------------------------------------------------------------
+            const int N = 100000;
+            var subject = new List<int>(N) {1};
+            for (int i = 1; i < N; i++)
+            {
+                subject.Add(2);
+            }
+
+            //-----------------------------------------------------------------------------------------------------------
+            // Act
+            //-----------------------------------------------------------------------------------------------------------
+            Action action = () =>
+            {
+                try { subject.Should().AllBeEquivalentTo(1); }
+                catch (Exception)
+                {
+                    // ignored, we only care about execution time
+                }
+            };
+
+            ////-----------------------------------------------------------------------------------------------------------
+            //// Assert
+            ////-----------------------------------------------------------------------------------------------------------
+            action.ExecutionTime().Should().BeLessThan(100.Seconds());
+        }
+
         [Fact]
         public void
             When_an_object_implements_multiple_IEnumerable_interfaces_but_the_declared_type_is_assignable_to_only_one_it_should_respect_the_declared_type