This code works:

[Theory, InlineAutoData(typeof(Order))]
public void Foo<T>(T order)
{
}

While this code throws an exception:

[Theory, InlineAutoData(typeof(Order))]
public void Foo<T>(T order, string s)
{
}

To me it looks more like a breaking change in Data Theories but I include the exception message in order to investigate further.

System.InvalidOperationException : An exception was thrown while getting data for theory OrderServiceTests.Foo:

Ploeh.AutoFixture.ObjectCreationException: AutoFixture was unable to create an instance from T, most likely because it has no public constructor. 

at Ploeh.AutoFixture.Kernel.TerminatingSpecimenBuilder.Create(Object request, ISpecimenContext context)   

at Ploeh.AutoFixture.Kernel.CompositeSpecimenBuilder.<>c__DisplayClass6.<Create>b__1(ISpecimenBuilder b) 

at System.Linq.Enumerable.WhereSelectListIterator`2.MoveNext()   

at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext() 

at System.Linq.Enumerable.<DefaultIfEmptyIterator>d__a5`1.MoveNext() 

at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source)   

at Ploeh.AutoFixture.Kernel.CompositeSpecimenBuilder.Create(Object request, ISpecimenContext context) 

at Ploeh.AutoFixture.Kernel.RecursionGuard.Create(Object request, ISpecimenContext context)   

at Ploeh.AutoFixture.Kernel.SpecimenContext.Resolve(Object request)

at Ploeh.AutoFixture.Kernel.SeedIgnoringRelay.Create(Object request, ISpecimenContext context)   

at Ploeh.AutoFixture.Kernel.CompositeSpecimenBuilder.<>c__DisplayClass6.<Create>b__1(ISpecimenBuilder b)   

at System.Linq.Enumerable.WhereSelectListIterator`2.MoveNext()   

at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()   

at System.Linq.Enumerable.<DefaultIfEmptyIterator>d__a5`1.MoveNext()   

at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source)   

at Ploeh.AutoFixture.Kernel.CompositeSpecimenBuilder.Create(Object request, ISpecimenContext context)   

at Ploeh.AutoFixture.Kernel.Postprocessor`1.Create(Object request, ISpecimenContext context)   

at Ploeh.AutoFixture.Kernel.CompositeSpecimenBuilder.<>c__DisplayClass6.<Create>b__1(ISpecimenBuilder b)   

at System.Linq.Enumerable.WhereSelectListIterator`2.MoveNext()   

at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()   

at System.Linq.Enumerable.<DefaultIfEmptyIterator>d__a5`1.MoveNext()   

at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source)   

at Ploeh.AutoFixture.Kernel.CompositeSpecimenBuilder.Create(Object request, ISpecimenContext context)   

at Ploeh.AutoFixture.Kernel.RecursionGuard.Create(Object request, ISpecimenContext context)   

at Ploeh.AutoFixture.Kernel.SpecimenContext.Resolve(Object request)   

at Ploeh.AutoFixture.Kernel.ParameterRequestRelay.Create(Object request, ISpecimenContext context)   

at Ploeh.AutoFixture.Kernel.CompositeSpecimenBuilder.<>c__DisplayClass6.<Create>b__1(ISpecimenBuilder b)   

at System.Linq.Enumerable.WhereSelectListIterator`2.MoveNext()   

at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext() 

at System.Linq.Enumerable.<DefaultIfEmptyIterator>d__a5`1.MoveNext()   

at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source)   

at Ploeh.AutoFixture.Kernel.CompositeSpecimenBuilder.Create(Object request, ISpecimenContext context)   

at Ploeh.AutoFixture.Kernel.Postprocessor`1.Create(Object request, ISpecimenContext context)   

at Ploeh.AutoFixture.Kernel.CompositeSpecimenBuilder.<>c__DisplayClass6.<Create>b__1(ISpecimenBuilder b)   

at System.Linq.Enumerable.WhereSelectListIterator`2.MoveNext()   

at System.Linq.Enumerable.WhereSelectEnumerableIterator`2.MoveNext()   

at System.Linq.Enumerable.<DefaultIfEmptyIterator>d__a5`1.MoveNext()   

at System.Linq.Enumerable.FirstOrDefault[TSource](IEnumerable`1 source)   

at Ploeh.AutoFixture.Kernel.CompositeSpecimenBuilder.Create(Object request, ISpecimenContext context)   

at Ploeh.AutoFixture.Kernel.RecursionGuard.Create(Object request, ISpecimenContext context)   

at Ploeh.AutoFixture.Kernel.SpecimenContext.Resolve(Object request)   

at Ploeh.AutoFixture.Xunit.AutoDataAttribute.Resolve(ParameterInfo p)   

at Ploeh.AutoFixture.Xunit.AutoDataAttribute.GetData(MethodInfo methodUnderTest, Type[] parameterTypes)   

at Ploeh.AutoFixture.Xunit.CompositeDataAttribute.<GetData>d__0.MoveNext()   

at Xunit.Extensions.TheoryAttribute.<GetData>d__7.MoveNext()   

at Xunit.Extensions.TheoryAttribute.EnumerateTestCommands(IMethodInfo method)

at Xunit.Extensions.TheoryAttribute.<>c__DisplayClass5.<EnumerateTestCommands>b__1()

at Xunit.Extensions.TheoryAttribute.LambdaTestCommand.Execute(Object testClass)

Does Order have a default constructor?

Yes, it has.

That's pretty strange because the error message claims that it doesn't. Looks like there's a bug somewhere...

Here is another example.

This works: 

[Theory, InlineAutoData("foo", "bar", "foo-bar")]
public void Foo<T>(T a, T b, T c)
{
}

However, the code below doesn't (both methods throw the same exception as above):

[Theory, InlineAutoData("foo", "bar")]
public void Foo<T>(T a, T b, T c)
{
}

[Theory, AutoData]
public void Foo<T>(T a, T b, T c)
{ 
}

Inside the AutoDataAttribute class, the code iterates through the parameters of the method under test. 

For each parameter it tries to create a specimen, however, the parameter is generic type so an exception is thrown because at this point there is no information on what type to create...

Oh, you're using the new xUnit.net 1.9 support for generics?

I am trying it, actually.

I think this is normal behavior.. (AutoData can't guess what the T is.)

Yes, that sounds reasonable. Have you tried this alternative (which has been available all along)?

AFAICT, this one applies to class level, though the generic theories of xUnit 1.9 apply to method level (which means, that the class itself may not be generic).

Since the behavior is normal, I think there is no action to be taken. :)