can somebody explain me what does “passing by value” and “Passing by reference” mean in C#?

问题

I am not quiet sure about the concept of "passing by value" and "passing by reference" in c#. I think Passing by value mean :

    int i = 9;

we pass int i into a method like: method(i) Passing by reference means exactly passing its location like :

    Class.method.variable

and it will give out the value. Can anybody help me out??

回答1:

In simple terms...

"Passing by value" means that you pass the actual value of the variable into the function. So, in your example, it would pass the value 9.

"Passing by reference" means that you pass the variable itself into the function (not just the value). So, in your example, it would pass an integer object with the value of 9.

This has various consequences, and each is useful in different situations.

This answer has more thorough information: What's the difference between passing by reference vs. passing by value?

回答2:

Few Points:

In C# unless, you use ref keyword, it is always Pass By Value.

Pass By Value: In case of Value Type - It is just another copy of Value Type Variable.

In case of Reference Type - It is a copy of reference (or pointer) to exact location where memory is assigned in heap.

Pass By Reference:

for Reference type variable - passing it to another method by using keyword is only useful in the scenario when the passed variable can be reassigned in the called method. It means :

let's say - Outside (caller) method A inside (called) method B

Reference Type Object R -passed to method B from method A then if you reassign the memory location of Object R in method B then those changes wouldn't be seen in Method A of Object R unless you pass the object R using ref keyword (i.e pass by reference).

回答3:

This is a very extensive subject so I will only try to answer your direct question. Passing something by value or reference to a method, will depend on what you're actually sending.

If it's a value type it will be sent by value, if it's a reference type it will send the reference by value. Basically it will send the variable itself to the method.

If you want true referencing, you have to use the ref keyword.

An example to showcase it:

void Main()
{
    int x = 5;
    Console.WriteLine (x);
    Test(x);
    Console.WriteLine (x);

    MyClass c = new MyClass();
    c.Name = "Jay";
    Console.WriteLine (c.Name);
    TestClass(c);
    Console.WriteLine (c.Name);
}

private void Test(int a){
 a += 5;
}

private void TestClass(MyClass c){
 c.Name = "Jeroen";
}

public class MyClass {
 public String Name {get; set; }
}

Output:

5
5
Jay
Jeroen

An int is a value type and thus its value is sent to the method. This means that any adjustment on that parameter will only be done on the local copy inside the method.

However, a class is a reference type and thus myClass will send its memory location. This means that by changing the data inside the given object, this will also change the value of this class in different scopes because they both refer to the same memory location.

If you'd use the ref keyword, the reference would be transmitted.

If you want more information, take it from a much more experienced person than me.

回答4:

Passing by value means you are passing the actual reference to the location in memory for that object. So if you have a class, something like:

CustomObj aObj = New CustomObj();
aObj.SomeString = "Test";
someMethod(aObj);

And a method header like so:

someMethod(CustomObj objIn){
     objIn.SomeString = "Changing by value";
}

And wrote out to the console:

Console.WriteLine(aObj.SomeString)

It would produce "Changing by value" instead of "Test" because we have a reference to that object.

By value, conversely, does not work this way.Keeping with our example above:

int byValue = 0;
someMeothd(byValue);

someMethod(int numIn){
   numIn++;
} 

Console.WriteLine(byValue);

Would produce "0".

来源：https://stackoverflow.com/questions/19505935/can-somebody-explain-me-what-does-passing-by-value-and-passing-by-reference

标签

reference

parameter-passing