为什么我的代码不更新数据库？

Question

look at these two codes

1. First:

    Slide _Slide = DataContext.Slides.SingleOrDefault(rec => rec.Id == _SlideObj.Id); _Slide = _SlideObj; DataContext.SaveChanges(); 

2. Second:

    Slide _Slide = DataContext.Slides.SingleOrDefault(rec => rec.Id == _SlideObj.Id); _Slide.Title = _SlideObj.Title; _Slide.Description = _SlideObj.Description; DataContext.SaveChanges(); 

The first code does not update the database but the second updates. Why? How can I change my code to do update operation in first method? (I like to use first method)

Answer 1

The first only changes the value of a variable to be a different reference. It doesn't actually update the object in any way. The data-context is tracking the object .

Answer 2

There is no update because you didn't change anything. In the first case, you just changed _Slide to point to a different Slide.

Answer 3

The problem of the first approach is by design and has nothing to do with the framework you're using.

By calling: _Slide = _SlideObj;
you are telling _Slide to point on _SlideObj instead, but in-memory, the Title and Description properties for the previous pointer to _Slider remain unchanged.

Therefore, this is normal behavior.