ContainItemsAssignableTo Now Expects At Least One Item Assignable to T

Src/FluentAssertions/Collections/GenericCollectionAssertions.cs

@@ -965,20 +965,18 @@ public AndConstraint<TAssertions> ContainItemsAssignableTo<TExpectation>(string
 
             if (success)
             {
-                int index = 0;
-                foreach (T item in Subject)
-                {
-                    if (item is not TExpectation)
-                    {
-                        Execute.Assertion
-                            .BecauseOf(because, becauseArgs)
-                            .FailWith(
-                                "Expected {context:collection} to contain only items of type {0}{reason}" +
-                                ", but item {1} at index {2} is of type {3}.", typeof(TExpectation), item, index, item.GetType());
-                    }
-
-                    ++index;
-                }
+                Execute.Assertion
+                    .BecauseOf(because, becauseArgs)
+                    .WithExpectation("Expected {context:collection} to contain element assignable to type {0}{reason}, ", typeof(TExpectation).FullName)
+                    .Given(() => Subject)
+                    .ForCondition(subject => subject.Any())
+                    .FailWith("but was empty.")
+                    .Then
+                    .ForCondition(subject => subject.Any(x => typeof(TExpectation).IsAssignableFrom(GetType(x))))
+                    .FailWith("but found {0}.",
+                        subject => $"[{string.Join(", ", subject.Select(x => GetType(x).FullName))}]")
+                    .Then
+                    .ClearExpectation();
             }
 
             return new AndConstraint<TAssertions>((TAssertions)this);

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

@@ -24,7 +24,7 @@ public void Should_succeed_when_asserting_collection_with_all_items_of_same_type
         }
 
         [Fact]
-        public void Should_fail_when_asserting_collection_with_items_of_different_types_only_contains_item_of_one_type()
+        public void Should_succeed_when_asserting_collection_with_items_of_different_types_only_contains_item_of_one_type()
         {
             // Arrange
             var collection = new List<object>
@@ -33,68 +33,50 @@ public void Should_fail_when_asserting_collection_with_items_of_different_types_
                 "2"
             };
 
-            // Act
-            Action act = () => collection.Should().ContainItemsAssignableTo<string>();
-
-            // Assert
-            act.Should().Throw<XunitException>();
+            // Act / Assert
+            collection.Should().ContainItemsAssignableTo<string>();
         }
 
         [Fact]
-        public void When_a_collection_contains_anything_other_than_strings_it_should_throw_and_report_details()
+        public void When_asserting_collection_contains_item_assignable_to_against_null_collection_it_should_throw()
         {
             // Arrange
-            var collection = new List<object>
-            {
-                1,
-                "2"
-            };
+            int[] collection = null;
 
             // Act
-            Action act = () => collection.Should().ContainItemsAssignableTo<string>();
+            Action act = () =>
+            {
+                using var _ = new AssertionScope();
+                collection.Should().ContainItemsAssignableTo<string>("because we want to test the behaviour with a null subject");
+            };
 
             // Assert
             act.Should().Throw<XunitException>().WithMessage(
-                "Expected collection to contain only items of type System.String, but item 1 at index 0 is of type System.Int32.");
+                "Expected collection to contain element assignable to type System.String because we want to test the behaviour with a null subject, but found <null>.");
         }
 
         [Fact]
-        public void When_a_collection_contains_anything_other_than_strings_it_should_use_the_reason()
+        public void When_a_collection_is_empty_an_exception_should_be_thrown()
         {
             // Arrange
-            var collection = new List<object>
-            {
-                1,
-                "2"
-            };
+            int[] collection = Array.Empty<int>();
 
             // Act
-            Action act = () => collection.Should().ContainItemsAssignableTo<string>(
-                "because we want to test the failure {0}", "message");
+            Action act = () => collection.Should().ContainItemsAssignableTo<int>();
 
             // Assert
-            act.Should().Throw<XunitException>()
-                .WithMessage(
-                    "Expected collection to contain only items of type System.String because we want to test the failure message" +
-                        ", but item 1 at index 0 is of type System.Int32.");
+            act.Should().Throw<XunitException>().WithMessage("Expected collection to contain element assignable to type \"System.Int32\", but was empty.");
         }
 
         [Fact]
-        public void When_asserting_collection_contains_item_assignable_to_against_null_collection_it_should_throw()
+        public void Should_throw_exception_when_asserting_collection_for_missing_item_type()
         {
-            // Arrange
-            int[] collection = null;
+            var collection = new object[] { "1", 1.0m };
 
-            // Act
-            Action act = () =>
-            {
-                using var _ = new AssertionScope();
-                collection.Should().ContainItemsAssignableTo<string>("because we want to test the behaviour with a null subject");
-            };
+            Action act = () => collection.Should().ContainItemsAssignableTo<int>();
 
-            // Assert
-            act.Should().Throw<XunitException>().WithMessage(
-                "Expected collection to contain element assignable to type System.String because we want to test the behaviour with a null subject, but found <null>.");
+            act.Should().Throw<XunitException>()
+                .WithMessage("Expected collection to contain element assignable to type \"System.Int32\", but found \"[System.String, System.Decimal]\".");
         }
 
         #endregion

docs/_pages/releases.md

@@ -12,6 +12,7 @@ sidebar:
 ### What's New
 
 ### Fixes
+* `ContainItemsAssignableTo` Now Expects At Least One Item Assignable to `T` - [#1765](https://github.com/fluentassertions/fluentassertions/pull/1765)
 
 ## 6.3.0