Code Inspection: NUnit. Test case source must be field, property, or method only

[TestFixture]
public sealed class TestCaseSourceTest
{
    static IEnumerable<int> _fieldSource = new[] {1, 2, 3};

    static IEnumerable<int> PropertySource => new[] {4, 5, 6};

    static IEnumerable<int> MethodSource() => new[] {7, 8, 9};

    public event UnhandledExceptionEventHandler OnError
    {
        add => throw new NotImplementedException();
        remove => throw new NotImplementedException();
    }

    [TestCaseSource(nameof(_fieldSource))] // ok
    [TestCaseSource(nameof(PropertySource))] // ok
    [TestCaseSource(nameof(MethodSource))] // ok
    [TestCaseSource(nameof(OnError))] // Warning: expected field/property/method
    public void Test1(int x)
    {
        Console.WriteLine(x);
    }
}