Mocking a method with calculated return value, instead of static return value. (Moq, TDD)

ReturnCalculatedValue.cs

namespace MoqNotes
{
    public interface ISubjectUnderTest
    {
        int GetData(int param);
    }

    public class Tests
    {
        [Fact]
        public void ReturnCalculatedValue()
        {
            //Arrange
            int valueToReturn = 0;

            Mock<ISubjectUnderTest> subjectUnderTestMock = new Mock<ISubjectUnderTest>();
            subjectUnderTestMock
                .Setup(x => x.GetData(It.IsAny<int>()))
                .Callback((int param) =>
                {
                    valueToReturn = ++param;
                })
                .Returns(()=> valueToReturn); //!!! NOT ".Returns(valueToReturn)", watch out for "()=>"

            //Act 
            ISubjectUnderTest subjectUnderTest = subjectUnderTestMock.Object;
            var result = subjectUnderTest.GetData(19);

            //Assert
            Assert.Equal(result, 20);
        }
    }

}