.NET Core 2.2: xUnit Theory Inlinedata not working with enum values

You don't need [MemberData], enum values should work right out of the box. As per the documentation enums are constants:

An enum type is a distinct value type (Value types) that declares a set of named constants.

Code example below works for me (a .net core 3.0 xUnit Test Project template):

public class UnitTest1
{
    public enum Foo { Bar, Baz, Qux }

    [Theory]
    [InlineData(Foo.Bar, Foo.Baz)]
    public void Test1(Foo left, Foo right)
    {
        Assert.NotEqual(left, right);
    }
}

Something else must be giving you troubles.

Attributes for [InlineData] need constant expressions, e.g int, bool, string etc.

Use [MemberData] instead if the inline is not recognizing the enum as a constant.

[Theory]
[MemberData(nameof(PeriodData))]
public void ShouldParsePeriod(string periodString, int value, PeriodUnit periodUnit) {
    var period = Period.Parse(periodString);
    period.Value.Should().Be(value);
    period.PeriodUnit.Should().Be(periodUnit);
}


public static IEnumerable<object[]> PeriodData() {
    yield return new object[] { "12h", 12, PeriodUnit.Hour };
    yield return new object[] { "3d", 3, PeriodUnit.Day };
    yield return new object[] { "1m", 1, PeriodUnit.Month };
}

Reference xUnit Theory: Working With InlineData, MemberData, ClassData