[release/10.0] Remove IList.Contains use from OfType

src/libraries/System.Linq/src/System/Linq/OfType.SpeedOpt.cs

@@ -177,15 +177,12 @@ public override IEnumerable<TResult2> Select<TResult2>(Func<TResult, TResult2> s
 
             public override bool Contains(TResult value)
             {
-                // Avoid checking for IList when size-optimized because it keeps IList
-                // implementations which may otherwise be trimmed. Since List<T> implements
-                // IList and List<T> is popular, this could potentially be a lot of code.
-                if (!IsSizeOptimized &&
-                    !typeof(TResult).IsValueType && // don't box TResult
-                    _source is IList list)
-                {
-                    return list.Contains(value);
-                }
+                // It is tempting to delegate here to IList.Contains if _source is IList (especially
+                // if TResult is not a value type, as it would be boxed as an argument to Contains).
+                // And while that will be correct in most cases, if any of the items in the source
+                // compares equally with value but is not actually of type TResult, doing so would
+                // skip the type check implied by OfType<TResult>(). Further, if IList is a multidim
+                // array, its IList.Contains will fail for non-1 ranks. We thus just iterate directly.
 
                 foreach (object? item in _source)
                 {

src/libraries/System.Linq/tests/OfTypeTests.cs

@@ -228,5 +228,55 @@ public void MultipleIterations()
                 Assert.Equal(i + 1, count);
             }
         }
+
+        [Fact]
+        public void MultiDimArray_OfType_Succeeds()
+        {
+            var array = new string[3, 4];
+            for (int i = 0; i < 3; i++)
+            {
+                for (int j = 0; j < 4; j++)
+                {
+                    array[i, j] = $"{i}{j}";
+                }
+            }
+
+            // ToArray
+            var result = array.OfType<string>().ToArray();
+            Assert.Equal(12, result.Length);
+            for (int i = 0; i < 3; i++)
+            {
+                for (int j = 0; j < 4; j++)
+                {
+                    Assert.Equal($"{i}{j}", result[i * 4 + j]);
+                }
+            }
+
+            // Contains
+            foreach (string s in array)
+            {
+                Assert.True(array.OfType<string>().Contains(s));
+            }
+        }
+
+        [Fact]
+        public void OfType_Contains_FiltersByTypeEvenIfEqual()
+        {
+            List<MySneakyObject1> list = [new MySneakyObject1()];
+            Assert.Empty(list.OfType<MySneakyObject2>());
+            Assert.False(list.OfType<MySneakyObject2>().Contains(new MySneakyObject2()));
+        }
+
+        private class MySneakyObject1
+        {
+            public override bool Equals(object? obj) => obj is MySneakyObject1;
+            public override int GetHashCode() => 0;
+        }
+
+        private class MySneakyObject2 : MySneakyObject1
+        {
+            public override bool Equals(object? obj) => obj is MySneakyObject2;
+            public override int GetHashCode() => 0;
+        }
     }
 }