MultiResponse.cs

    // could be struct?
    public class MultiResponse<T, E> {
        // could be struct?
        public class OkResponse: MultiResponse<T,E> {
            public T value;
        }
        
        public class ErrResponse: MultiResponse<T,E> {
            public E value;
        }

        public static MultiResponse<T, E> Ok(T value) {
            return new OkResponse { value = value };
        }
        public static MultiResponse<T,E> Err(E value) {
            return new ErrResponse { value = value };
        }
    }

test.cs

namespace MassTransit.Tests
{
    using MassTransit;

    [TestFixture]
    public class MultiResponseTest
    {
        [Test]
        public void CanMatch() {
            var good = MultiResponse<string, int>.Ok("hi");
            var bad = MultiResponse<string, int>.Err(1);

            switch(good) {
                case MultiResponse<string, int>.OkResponse r:
                    return;
                case MultiResponse<string, int>.ErrResponse e:
                    return;
            }
        }
    }
}