论immutable不可变性

In Java strings are immutable. If we have a string and make changes to it, we get new string referenced by the same variable:

String str = "abc";
str += "def"; // now str refers to another piece in the heap containing "abcdef"
              // while "abc" is still somewhere in the heap until taken by GC

It‘s been said that int and double are immutable in C#. Does it mean that when we have int and later change it, we would get new int "pointed" by the same variable? Same thing but with stack.

int i = 1;
i += 1; // same thing: in the stack there is value 2 to which variable
        // i is attached, and somewhere in the stack there is value 1

Is that correct? If not, in what way is int immutable?

You haven‘t changed (and cannot change) something about the int; you have assigned a new int value (and discarded the old value). Thus it is immutable.

Consider a more complex struct:

var x = new FooStruct(123);
x.Value = 456; // mutate
x.SomeMethodThatChangedInternalState(); // mutate

x = new FooStruct(456); // **not** a mutate; this is a *reassignment*

However, there is no "pointing" here. The struct is directly on the stack (in this case): no references involved.