AutoFixture：解析 IEnumerable<t> ，无法将“ConvertedEnumerable”类型的 object 转换为列表</t>

Question

I tried to use AutoFixture to generate test data for classes that have IEnumerable members.

AutoFixture test code:

var fixture = new Fixture();
fixture.Customize<NotificationDto>(c => c.Without(r => r.IntegrationModel));
NotificationDto notificationDto = fixture.Create<NotificationDto>();

Class:

public class NotificationDto 
{
    public NotificationDto() 
    {
        To = new List<RecipientDto>();
    }
    public Guid Id { get; set; }
    public IIntegrationEventModel IntegrationModel { get; set; }
    public IEnumerable<RecipientDto> To { get; set; }
}

During debugging of code

 public CreateNotificationCommand(NotificationDto notificationDto)
    {
        ...
        To = (List<RecipientDto>)(notificationDto.To ?? new List<RecipientDto>());
    }

Getting below error:

 UpdateNotificationFromAwsSnsCommandHandlers_UpdateNotificationToSuccess
   Duration: 8.9 sec

  Message: 
    System.InvalidCastException : Unable to cast object of type 'ConvertedEnumerable`1[Notification.Core.Notification.Dto.SolrDto.RecipientSolrDto]' to type 'System.Collections.Generic.List`1[Notification.Core.Notification.Dto.SolrDto.RecipientSolrDto]'.

  Stack Trace: 
    CreateNotificationCommand.ctor(NotificationDto notificationDto) line 31
    UpdateNotificationFromAwsSnsCommandHandlersTester.SetAutoData_UpdateNotificationFromAwsSnsCommandHandlers_UpdateNotificationToNotDelivered() line 60
    UpdateNotificationFromAwsSnsCommandHandlersTester.UpdateNotificationFromAwsSnsCommandHandlers_UpdateNotificationToSuccess() line 75
    GenericAdapter`1.GetResult()
    AsyncToSyncAdapter.Await(Func`1 invoke)
    TestMethodCommand.RunTestMethod(TestExecutionContext context)
    TestMethodCommand.Execute(TestExecutionContext context)
    c__DisplayClass1_0.b__0()
    BeforeAndAfterTestCommand.RunTestMethodInThreadAbortSafeZone(TestExecutionContext context, Action action)
 

Please help me?

Answer 1

To is an IEnumerable , so it is not guaranteed to be a List , so this cast is not guaranteed to work (in your unit test AutoFixture does not create a list, for example). You can change your code to:

To = notificationDto.To ?? Enumerable.Empty<RecipientDto>();

or something like:

To = notificationDto.To as List<RecipientDto> 
    ?? notificationDto.To?.ToList() 
    ?? new List<RecipientDto>();

Or change the signature of To to List<RecipientDto> .

Answer 2

The exception is thrown because notificationDto.To isn't necessarily List<RecipientDto> at runtime, it can be an array of RecipientDto or any other type implicitly casted to IEnumerable<RecipientDto> . Hence, when notificationDto.To has different runtime type from List<RecipientDto> the InvalidCastException is thrown.

To fix this you can either just remove casting to the list and everything should work fine or have Enumerable.Empty<RecipientDto>() on the right side of the operator ( for both only if To on the right side of = is IEnumerable<RecipientDto> ):

1. To = notificationDto.To ?? new List<RecipientDto>();
2. To = notificationDto.To ?? Enumerable.Empty<RecipientDto>();

The first approach works because the result type of the expression in this case will be IEnumerable<RecipientDto> . This follows from the specification for the operator ?? https://learn.microsoft.com/en-us/do.net/csharp/language-reference/language-specification/expressions#1114-the-null-coalescing-operator .

We're interested in this part of the spec:

if A is not a nullable type ( but a reference type ) and an implicit conversion exists from b to A , the result type is A . At run-time, a is first evaluated. If a is not null , a becomes the result. Otherwise, b is evaluated and converted to type A , and this becomes the result.`

Where A is the type of a .

In your case a is notificationDto.To and b is new List<RecipientDto>() , therefore A is IEnumerable<RecipientDto> . Since IEnumerable<RecipientDto> is not a nullable type and there is a conversion from new List<RecipientDto> to IEnumerable<RecipientDto> the result type of the expression will be also IEnumerable<RecipientDto> . Hence if To is already IEnumerable<RecipientDto> you don't need to do any casting.