How to exclude certain enumerations from all enumeration types

autofixture, c#

Solution

You can create a custom Specimen Builder that uses the existing `EnumGenerator` but skips the values you don't need. In this specimen I use the `specimen.ToString()` method intentionally in order not to be linked to any particular `enum` type:

public class ExcludeUnknownEnumGenerator : ISpecimenBuilder
{
    private readonly EnumGenerator _enumGenerator = new EnumGenerator();

    public object Create(object request, ISpecimenContext context)
    {
        var enumType = request as Type;
        if (enumType == null || !enumType.IsEnum)
        {
            return new NoSpecimen();
        }

        var namesEnumerator = Enum.GetNames(enumType).GetEnumerator();
        while (namesEnumerator.MoveNext())
        {
            var specimen = _enumGenerator.Create(request, context);
            if (specimen.ToString() != "Unknown" &&
                specimen.ToString() != "Uninitialized")
            {
                return specimen;
            }
        }

        throw new ObjectCreationException(
            "AutoFixture was unable to create a value for " +
            enumType.FullName +
            " since it is an enum containing either no values or " +
            "ignored values only ('Unknown' and 'Uninitialized'). " +
            "Please add at least one valid value to the enum.");
    }
}

public enum EnumWithUnknown
{
    Known,
    Unknown,
    Wellknown,
    Uninitialized
}

The following code shows `enum` values without the `Unknown` and `Uninitialized` states:

var fixture = new Fixture();
fixture.Customizations.Insert(0, new ExcludeUnknownEnumGenerator());

Console.Out.WriteLine("result = {0}", fixture.Create<EnumWithUnknown>());
Console.Out.WriteLine("result = {0}", fixture.Create<EnumWithUnknown>());
Console.Out.WriteLine("result = {0}", fixture.Create<EnumWithUnknown>());
Console.Out.WriteLine("result = {0}", fixture.Create<EnumWithUnknown>());

*** ConsoleOutput ***

result = Known
result = Wellknown
result = Known
result = Wellknown

Problem

I am trying to exclude certain enumeration values such as `Unknown` and `Uninitialized` from the set of values for any enumeration type. I can see that `Enums` are generated in a round robin fashion using the `EnumGenerator` from the set of all possible `Enum` values for a given `Enum` type. Based on that code, my first thought is to build an `ISpecimenBuilder` that checks for `Type.IsEnum` and does a `context.Resolve(request)` until `Resolve` returns a value that is not on the excluded list. The problem is this gives me a recursion error. After inspecting the source code I understand why - if a builder handles a request and calls another `Resolve` again with the same request you will end up in an infinite loop. But as `EnumGenerator` is not extensible, and I can't figure out how to intercept the build chain I am stumped as to how to solve this.

Original source