AutoFixture is unable to create an instance of a class that has a property of type RestSharp.RestResponse

Question

I am attempting to use AutoFixture to create instances of a class with dummy values for use in my unit-tests. One class in particular is giving me trouble, and it is a class that has a property of the type "RestResponse" which comes from the "RestSharp" library. When I run the code, I receive one of the two errors I have attached. The frequency of these errors is seemingly random.

The code I am using with a simplified version of the class:

using AutoFixture;
using AutoFixture.AutoMoq;
using RestSharp;

var fixture = new Fixture()
    .Customize(new AutoMoqCustomization
    {
        ConfigureMembers = true,
    });

var t = fixture
    .Build<C>()
    .Create();
    
class C
{   
    public RestResponse MyResponse { get; set; }
}

At times I receive this error:

AutoFixture was unable to create an instance from System.Net.CookieContainer because creation unexpectedly failed with exception. Please refer to the inner exception to investigate the root cause of the failure.

Request path:
    RestSharp.RestResponse MyResponse
      RestSharp.RestResponse
        RestSharp.RestRequest Request
          RestSharp.RestRequest
            System.Net.CookieContainer CookieContainer
              System.Net.CookieContainer

Inner exception messages:
    System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
      System.ArgumentOutOfRangeException: value ('205') must be less than or equal to '38'. (Parameter 'value')
Actual value was 205.

At times I also receive this error instead of the previous error:

AutoFixture was unable to create an instance from Moq.Mock`1[System.IO.Stream] because creation unexpectedly failed with exception. Please refer to the inner exception to investigate the root cause of the failure.

Request path:
    RestSharp.RestResponse MyResponse
      RestSharp.RestResponse
        RestSharp.RestRequest Request
          RestSharp.RestRequest
            System.Func`2[System.IO.Stream,System.IO.Stream] ResponseWriter
              System.Func`2[System.IO.Stream,System.IO.Stream]
                System.IO.Stream
                  Moq.Mock`1[System.IO.Stream]

Inner exception messages:
    System.ArgumentException: GenericArguments[0], 'System.Span`1[System.Byte]', on 'TValue IsAny[TValue]()' violates the constraint of type 'TValue'.
      System.Security.VerificationException: Method Moq.It.IsAny: type argument 'System.Span`1[System.Byte]' violates the constraint of type parameter 'TValue'.

If you have any suggestions, please feel free to share with me. Thank you for your help.

Answer 1

I think that you have two separate issues here. I'll address the first.

It appears to be that the CookieContainer type has one, or more properties that is dependent on, and constrained by the value of another property in the type. When it's creating the CookieContainer type, AutoFixture creates 'anonymous' values for primitive types like int with no determinism, so the first value it creates could be greater than or less than the previous value it's created. This is a problem when the second property AutoFixture is trying to set is validated against the first.

You could constrain AutoFixture and inject a known int value that you know will satisfy the validation, but this is a brittle approach that might not satisfy all of the validation rules in the CookieContainer type and could trigger problems elsewhere.

It's probably better to tell AutoFixture how to create a reasonably well-formed instance of the CookieContainer, using sensible default values. You can do this by injecting an instance of the CookieContainer you've created or use the ISpecimenBuilder approach.