public static void EnsureBadRequestObjectResultForError(this IActionResult actionResult, String expectedErrorKey, String expectedErrorMsg) { SerializableError content = actionResult.EnsureBadRequestObjectResult <SerializableError>(); Assert.True(content.ContainsKey(expectedErrorKey)); Object rawElement = content[expectedErrorKey]; Assert.IsAssignableFrom <IEnumerable <String> >(rawElement); IEnumerable <String> errorElements = (IEnumerable <String>)rawElement; Assert.Single(errorElements); Assert.Equal(expectedErrorMsg, errorElements.First()); }