Prevent multiple enumeration in ContainSingle()

Src/FluentAssertions/Collections/GenericCollectionAssertions.cs

@@ -1005,15 +1005,16 @@ public AndConstraint<TAssertions> ContainItemsAssignableTo<TExpectation>(string
 
             if (success)
             {
-                switch (Subject.Count())
+                ICollection<T> actualItems = Subject.ConvertOrCastToCollection();
+                switch (actualItems.Count)
                 {
                     case 0: // Fail, Collection is empty
                         Execute.Assertion
                             .BecauseOf(because, becauseArgs)
                             .FailWith("Expected {context:collection} to contain a single item{reason}, but the collection is empty.");
                         break;
                     case 1: // Success Condition
-                        match = Subject.SingleOrDefault();
+                        match = actualItems.Single();
                         break;
                     default: // Fail, Collection contains more than a single item
                         Execute.Assertion

Tests/FluentAssertions.Specs/Collections/CollectionAssertionSpecs.ContainSingle.cs

@@ -246,6 +246,34 @@ public void When_single_item_is_found_it_should_allow_continuation()
             act.Should().Throw<XunitException>().WithMessage(expectedMessage);
         }
 
+        [Fact]
+        public void When_collection_is_IEnumerable_it_should_be_evaluated_only_once_with_predicate()
+        {
+            // Arrange
+            int evalCount = 0;
+            IEnumerable<int> collection = new[] { 1 }.Select(_ => evalCount++);
+
+            // Act
+            collection.Should().ContainSingle(_ => true);
+
+            // Assert
+            evalCount.Should().Be(1);
+        }
+
+        [Fact]
+        public void When_collection_is_IEnumerable_it_should_be_evaluated_only_once()
+        {
+            // Arrange
+            int evalCount = 0;
+            IEnumerable<int> collection = new[] { 1 }.Select(_ => evalCount++);
+
+            // Act
+            collection.Should().ContainSingle();
+
+            // Assert
+            evalCount.Should().Be(1);
+        }
+
         #endregion
     }
 }

docs/_pages/releases.md

@@ -13,6 +13,7 @@ sidebar:
 * Adding `ThatAreAsync()` and `ThatAreNotAsync()` for filtering in method assertions - [#1725](https://github.com/fluentassertions/fluentassertions/pull/1725)
 * Adding `ThatAreVirtual()` and `ThatAreNotVirtual()` for filtering in method assertions - [#1744](https://github.com/fluentassertions/fluentassertions/pull/1744)
 ### Fixes
+* Prevent multiple enumeration of `IEnumerable`s in parameter-less `ContainSingle()` - [#1752](https://github.com/fluentassertions/fluentassertions/issues/1752)
 
 ## 6.2.0
 
@@ -27,7 +28,7 @@ sidebar:
 * `At` now retains the `DateTimeKind` and keeps sub-second precision when using a `TimeSpan` - [#1687](https://github.com/fluentassertions/fluentassertions/pull/1687).
 * Removed iteration over enumerable when generating the `BeEmpty` assertion failure message - [#1692](https://github.com/fluentassertions/fluentassertions/pull/1692).
 * Prevent `ArgumentNullException` when formatting a lambda expression containing an extension method - [#1696](https://github.com/fluentassertions/fluentassertions/pull/1696)
-* `IgnoringCyclicReferences` in `BeEquivalentTo` now works while comparing value types using `ComparingByMembers` - [#1708](https://github.com/fluentassertions/fluentassertions/pull/1708) 
+* `IgnoringCyclicReferences` in `BeEquivalentTo` now works while comparing value types using `ComparingByMembers` - [#1708](https://github.com/fluentassertions/fluentassertions/pull/1708)
 * Using `BeEquivalentTo` on a collection with nested collections would complain about missing members - [#1713](https://github.com/fluentassertions/fluentassertions/pull/1713)
 * Formatting a lambda expression containing lifted operators - [#1714](https://github.com/fluentassertions/fluentassertions/pull/1714).
 * Performance improvements in `BeEquivalentTo` by caching expensive Reflection operations - [#1719](https://github.com/fluentassertions/fluentassertions/pull/1719)