DefaultValue.Empty vs "enumerables", and the use of SetReturnsDefault

[pjank]

Describe the Bug

Firstly, DefaultValue.Empty is described as: "Default behavior, which generates ... empty array and enumerables ...."
It may be a matter of semantics but I expected something like a List<T> to be also treated as "enumerables" in this context. Depending on point of view, applying this to everything implementing IEnumerable<T> may be too much (should a string be "empty" or "null" in this mode?) but how about everything related to ICollection<T>?
If nothing else, I hope this post will help others who encounter the same issue and wonder how to "make it work" (without explicit Setup for each such method on a mock).

Secondly, SetReturnsDefault - it's currently very "picky" ;)
It would be nice, IMHO, if it automatically supported all derived types... and/or wrapped in a Task, ValueTask.

Steps to Reproduce

This (MSTest-style) test code should explain it all:

[TestClass]
public class DefaultValue_Empty_Enumerables
{
    public interface IFoo
    {
        int[] GetArray();
        IEnumerable<int> GetIEnumerable();
        ICollection<int> GetICollection();
        IList<int> GetIList();
        List<int> GetList();

        Task<int> GetArrayAsync();
        Task<List<int>> GetListAsync();
    }

    [TestMethod]
    public void AllThoseTypesCanBeCalled_Enumerables()
    {
        Assert.IsTrue(typeof(int[]).IsAssignableTo(typeof(IEnumerable<int>)));
        Assert.IsTrue(typeof(ICollection<int>).IsAssignableTo(typeof(IEnumerable<int>)));
        Assert.IsTrue(typeof(IList<int>).IsAssignableTo(typeof(IEnumerable<int>)));
        Assert.IsTrue(typeof(List<int>).IsAssignableTo(typeof(IEnumerable<int>)));

        Assert.IsTrue(typeof(string).IsAssignableTo(typeof(IEnumerable<char>)), "something to watch out for");
        Assert.IsFalse(typeof(string).IsAssignableTo(typeof(ICollection<char>)), "a more specific workaround?");
    }

    [TestMethod]
    public void UsingDefaultMoqSettings_DefaultValue_Empty()
    {
        var mock = new Mock<IFoo>();
        Assert.AreEqual(DefaultValue.Empty, mock.DefaultValue);
    }

    [TestMethod]
    public async Task MethodsWithTheseReturnTypes_DefaultToEmpty()
    {
        var mock = new Mock<IFoo>();
        Assert.IsNotNull(mock.Object.GetArray());
        Assert.IsNotNull(mock.Object.GetIEnumerable());
        Assert.IsNotNull(await mock.Object.GetArrayAsync());
    }

    [TestMethod]
    public async Task MethodsWithTheseReturnTypes_DefaultToNull()
    {
        var mock = new Mock<IFoo>();
        Assert.IsNull(mock.Object.GetICollection());
        Assert.IsNull(mock.Object.GetIList());
        Assert.IsNull(mock.Object.GetList());
        Assert.IsNull(await mock.Object.GetListAsync());
    }

    [TestMethod]
    public async Task MakingItAllWork_Almost()
    {
        var mock = new Mock<IFoo>();
        mock.SetReturnsDefault<ICollection<int>>([]);
        mock.SetReturnsDefault<IList<int>>([]);
        mock.SetReturnsDefault<List<int>>([]);

        Assert.IsNotNull(mock.Object.GetICollection());
        Assert.IsNotNull(mock.Object.GetIList());
        Assert.IsNotNull(mock.Object.GetList());
        Assert.IsNull(await mock.Object.GetListAsync());
    }

    [TestMethod]
    public async Task MakingItAllWork_IncludingAsync()
    {
        var mock = new Mock<IFoo>();
        mock.SetReturnsDefault<ICollection<int>>([]);
        mock.SetReturnsDefault<IList<int>>([]);
        mock.SetReturnsDefault<List<int>>([]);
        mock.SetReturnsDefault(Task.FromResult(new List<int>()));

        Assert.IsNotNull(mock.Object.GetICollection());
        Assert.IsNotNull(mock.Object.GetIList());
        Assert.IsNotNull(mock.Object.GetList());
        Assert.IsNotNull(await mock.Object.GetListAsync());
    }

    [TestMethod]
    public void BTW_WatchOutForReset_ItResetsTheDefaultSettingsAsWell()
    {
        var mock = new Mock<IFoo>();
        mock.SetReturnsDefault<List<int>>([]);
        Assert.IsNotNull(mock.Object.GetList());

        mock.Reset();
        Assert.IsNull(mock.Object.GetList());

        mock.SetReturnsDefault<List<int>>([]);
        Assert.IsNotNull(mock.Object.GetList());
    }
}

Version Info

Moq v4.20.70