what's means of value type and referance type in .net

Question

您好，我有点困惑valu类型或引用类型...例如，int是一个值类型字符串是引用类型，但是int也引用了一些值，例如int int i=4 int j=i这里j也被引用为4就像我为什么不将其称为引用类型...在string s="hello" string s1=s s是引用s .....任何人都可以从存储的角度进行解释。 .....对值类型或引用类型是否有任何限制。

Answer 1

int i=4;
int j=i;

int is value type, it means that this expression j=i copies value from i to j.

string s="hello";
string s1=s;

string is reference type so here not value is copied but reference to that string, so they now point to same object.

Answer 2

A value type holds a value, a reference type holds a reference to another variable.

In your example, i holds the value 4, but j=i means j still holds the value 4, even if i changes to 5. j doesn't change it's value, so it's not a reference type.

Strings are the same way. You can say s1 = s, but when s changes to 'hello2,' s1 stays 'hello'. Acts like a value type, but under the hood, stored like a reference type, see below.

Arrays and objects are reference types. If you say obj2 = obj, and obj changes, obj2 also changes. Obj2 holds a reference, or pointer to, obj, changes and all.

also see: Visual Basic .NET Syntax Reference Primer

containing:

A string is initialized as follows:

Dim myString as String = "Hello, World!"

Strings are Immutable, meaning once you assign a value to them they cannot be changed. Whenever you assign another value to a string or concatenate a value to a string, you are actually creating a new copy of a string variable and deleting the old copy of the string. (This has potential performance considerations if you are doing repeated work on a string variable, use the StringBuilder for such tasks as it is not immutable ie. The StringBuilder is Mutable).

I think it's more accurate to say you're deleting the old reference to the string variable, as any other strings pointing to it retain their value.

Answer 3

Ask yourself what happens when things change:

int i = 4;
int j = i;

i = 7;  // Q1). now what value does j have?

j = 99; // Q2) what value does i now have?

Q1). assigning 7 to i has no effect on j, j still has the value 4.

Q2). assigning 99 to j has no effect on i, i still has the (latest) value 7;

Strings are a bit tricker to explain, because in some languages you can't change them. So I'll use an (invented) language where it's legal to change values in a string.

string s1 = "Hello";
si[0] = 'J';  // now s1 is set to "Jello"

So we say

string s1 = "wibble";
string s2 = s1;
// here s2 is **referring** to "wibble", the same "wibble" that s1 refers to.

s1[1] = 'o';  

// now what does s1 refer to? "wobble"

// Big question: what does s2 refer to? The very same thing, "wobble".

So s1 and s2 are references to the same thing at the moment. We could now write

s1 = "SomethingElse";

Now we have chnaged the reference in s1, but s2 still refers to "wobble"