Do synonyms exist? | 易学教程

问题

Is there a way to have a variable set to an object and have another variable which is always equals to the former one?

var x = new object();
var y = [synonym of x];
x = null; // then y = null as well

I don't think this exists.

So I've often used arrays to hold "references".

var x = new object[] { new object() };
var y = x;
x[0] = null; // then y[0] = null as well

But it feels kinda lame.

回答1:

You can do it but the price you have to pay is to use undocumented keywords/features. They're there from long time ago and probably they won't change or disappear but...

It'll make your code more complicated to read (it may be useful if supported by the language itself) but it's bidirectional and you can move around your original object, changes will always be reflected to your "reference" too. It differs from Mehmet Ataş answer because you can pass the original object to another function and changes will propagate to your synonym too. They have limitations (they can't be used for class fields) but they works for parameters and local variables.

What you need is a TypedReference, it holds a reference to another variable then if you assign a new value to it you'll change the original variable. This in theory could open the door to synonyms if someday they'll think it's a good feature to include.

Let's see an example:

var text = "initial value";
var synonym = __makeref(text);

Now synonym is a reference to text (please note it's a reference to text and not to the value it holds). To get original value from a TypedReference you use __refvalue like this:

Console.WriteLine(__refvalue(synonym, string));

They have the same value:

Debug.Assert(__refvalue(synonym, string) == text);

Now let's change text to a new value:

text = "second value";
Debug.Assert(__refvalue(synonym, string) == text);

And even opposite is true:

__refvalue(synonym, string) = "third value"; // <---
Debug.Assert(__refvalue(synonym, string) == text);

Finally let's modify the original variable within another function (unaware of the reference it'll see a normal variable):

void ChangeIt(ref string value) { value = "another value"; }

ChangeIt(ref text);
Debug.Assert(__refvalue(synonym, string) == text);

All of this works will value types as well. Note that this creates a synonym for a variable, not an alias (you can imagine them as a safe pointer - to pointer in case of reference type). Let's try this:

void foo1()
{
    string text = "ABC";
    foo2(text);

    // text holds the original value "ABC"
    // not the value modified in foo2
}

void foo2(string value)
{
   value = "123";
   var synonym = __makeref(value);
   __refvalue(value, string) = "456";

   // both value and synonym holds "456";
}

回答2:

If you really really need to this you can do something like below but I think it is still lame (:

class RefHolder<T>
{
    public RefHolder(T value)
    {
        Value = value;
    }

    public T Value { get; set; }
}

Usage

var o1 = new RefHolder<object>(new object());
var o2 = o1;
o2.Value = null; // now o1.Value is null too

回答3:

Well, you are basicly describing a C++ reference (or a C pointer). This can be done in C#, too, but you REALLY do not want to do this unless you absolutely need to.

unsafe static void Main(string[] args)
    {
        int a = 5;

        int *b = &a;
        *b = 0;

        Console.WriteLine(a);
    }

This will output 0 to the console.

You can read more about unsafe code in Unsafe Code and Pointers article on MSDN.

回答4:

It depends. .NET contains both Reference and Value types. Value types are all the basic types, int, bool etc.. plus string. Reference types are everything else, including anything you create for yourself.

So, for example, value types...

int a = 3;
int b = a;
b = 5;
// a is still 3

While with references

class Mine {
  public int A { get; set; }
}
Mine A = new Mine() { A = 3; }
Mine B = A;
B.A = 5;
// A.A is now 5.

回答5:

you can asign like

var parentObject={};

parentobject['child1']="test1";
parentobject['child2']="test2";
parentobject['child3']="test3";

after

console.log(parentObject);

you get following output

object{child1="test1",child2="test2",child2="test2"}

来源：https://stackoverflow.com/questions/17364273/do-synonyms-exist

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