IFoo4.cs

[Test]
        public void OutAndRefTest()
        {
            var mock = new Mock<IFoo>();

            var requiredOutput = "ok";

            mock.Setup(foo => foo.TryParse("ping", out requiredOutput)).Returns(true);

            string result;

            Assert.Multiple(() =>
            {
                Assert.IsTrue(mock.Object.TryParse("ping",out result));
                Assert.That(result,Is.EqualTo(requiredOutput));
            });
     }