对象是引用类型还是值类型？

Question

I have still doubts about object . It is the primary base class of anything, any class. But is it reference type or value type. Or like which of these acts it? I need to get this clarified. I have difficulty understanding that.

     object obj1 = "OldString";
     object obj2 = obj1;
     obj1 = "NewString";
     MessageBox.Show(obj1 + "   " + obj2);
     //Output is  "NewString   OldString" 

In this case it acts like a value type. If object was reference type then why obj2 value is still "OldString"

   class SampleClass
    {
        public string Text { get; set; }
    }

    SampleClass Sample1 = new SampleClass();
    Sample1.Text="OldText";         

    object refer1 = Sample1;
    object refer2 = refer1;

    Sample1.Text = "NewText";

    MessageBox.Show((refer1 as SampleClass).Text +  (refer2 as SampleClass).Text);
    //OutPut is "NewText   NewText"   

In this case it acts like reference type

We can deduce that object 's type is what you box inside it. It can be both a reference type and value type. It is about what you box inside. Am I right?

Answer 1

It is a reference type

Doing an example with string isn't very illuminating, because string is also a reference type (as is SampleClass , obviously); your example contains zero "boxing".

if object is reference type then why obj2 value is still "OldString"

Why wouldn't it be? When you create a new string , that doesn't change old references to point at the new string. Consider:

 object obj1 = "OldString";
 // create a new string; assign obj1 the reference to that new string "OldString"

object obj2 = obj1;
 // copy the reference from obj1 and assign into obj2; obj2 now refers to
 // the same string instance

 obj1 = "NewString";
 // create a new string and assign that new reference to obj1; note we haven't
 // changed obj2 - that still points to the original string, "OldString"

Answer 2

When you do

obj1 = "NewString";

it actually holds a new reference, to another memory location, not the same location you gave to obj2 before. When you change the content of the location obj1 , you will get the same change in obj2 .

Try to change the content of obj1 with

fixed(char* c = obj1 as string)
{
    c = '0';
}

Both of your strings will now be "0ldString" .

This is because objects are reference types.

Answer 3

An object variable is always a reference-type.

It's possible for object to "reference" a value-type by the power of boxing. The box is a reference-type wrapper around a value, to which the object variable refers.

int x = 10;     // a value-type
object o = x;

The variable o is a reference to a box containing the value of x - but it's not x :

x = 20;
MessageBox.Show( string.Format( "x:{0} o:{1}", x, o ) );

This might be more illuminating with a mutable value-type:

struct SampleClass
{
    public string Text { get; set };
    public override string ToString() { return Text; }
}

var x = new SampleClass{ Text = "Hello" };
object o = x;
x.Text = "World";
MessageBox.Show( string.Format( "{0} {1}", x, o ) );

o is a boxed reference to x , so changing x 's value has no effect on o .

Changing SampleClass to be a class instead of a struct (reference-type instead of value-type) would change the behaviour: the line object o = x; would make o refer to the same thing as x, and changing x's text would also change o's text.

Answer 4

An object variable is always a reference-type. Classes and string are reference type. Struct and enum are kind of value types. I have put together a big example from various resources.

// PrintedPage is a value type
//this is a struct
struct PrintedPage
{
    public string Text;
}

// WebPage is a reference type
class WebPage
{
    public string Text;
}

struct SampleClass
{
    public string Text { get; set; }
    public override string ToString() { return Text; }
}

void Main()
{
        // First look at value type behaviour
        PrintedPage originalPrintedPage = new PrintedPage();
        originalPrintedPage.Text = "Original printed text";

        // Copy all the information
        PrintedPage copyOfPrintedPage = originalPrintedPage;

        // Change the new copy
        copyOfPrintedPage.Text = "Changed printed text";

        // Write out the contents of the original page.
        // Output=Original printed text
        Console.WriteLine ("originalPrintedPage={0}",
                           originalPrintedPage.Text);


       //-------------------------------------------------------------------
        // Now look at reference type behaviour
        WebPage originalWebPage = new WebPage();
        originalWebPage.Text = "Original web text";

        // Copy just the URL
        WebPage copyOfWebPage = originalWebPage;
        // Change the page via the new copy of the URL
        copyOfWebPage.Text = "Changed web text";

        // Write out the contents of the page
        // Output=Changed web text
        Console.WriteLine ("originalWebPage={0}",
                           originalWebPage.Text);

        // Now change the copied URL variable to look at
        // a different web page completely
        copyOfWebPage = new WebPage();
        copyOfWebPage.Text = "Changed web page again";

         Console.WriteLine ("originalWebPage={0}",
                           originalWebPage.Text);
        Console.WriteLine ("copyOfWebPage={0}",
                           copyOfWebPage.Text);


       //-------------------------------------------------------------------
        //string are reference type too
         object obj1 = "OriginalString"; // create a new string; assign obj1 the reference to that new string "OriginalString"
         object obj2 = obj1;// copy the reference from obj1 and assign into obj2; obj2 now refers to // the same string instance
         obj1 = "NotOriginalString";// create a new string and assign that new reference to obj1; note we haven't // changed obj2 - that still points to the original string, "OriginalString"
        /*   When you do obj1 = "NewString"; it actually holds a new reference, to another memory location, not the same location you gave to obj2 before. 
           IMP -  When you change the content of the location obj1, you will get the same change in obj2.
        */
         Console.WriteLine(obj1 + "   " + obj2);

       //-------------------------------------------------------------------
         object onj11 = 2; 
         object obj12 = onj11;
         onj11 = 3; //you assigned boj11 to a new reference but obj12 reference did not change
         Console.WriteLine(onj11 + "   " + obj12);

       //-------------------------------------------------------------------     
         /*look below - it's possible for object to "reference" a value-type by the power of boxing. The box is a reference-type wrapper around a value, to which the object variable refers.*/
         int i = 2; //int is value type
         object j = i; //variable j is a reference to a box containing the value of i- but it's not i
         i = 3;  
         Console.WriteLine(i + "   " + j);       

       //-------------------------------------------------------------------
        var x = new SampleClass{ Text = "Hello" };
        object o = x;
        x.Text = "World";
        Console.WriteLine(x.Text + "   " + o);

       //-------------------------------------------------------------------
        SampleClass x1 = new SampleClass{ Text = "Hello" }; //sample class is of type struct which is value type; it is was of type class then the data would be copied over and result would be World World
        SampleClass o1 = x1;
        o1.Text = "World";
        Console.WriteLine(x + "   " + o);
    }

References - http://jonskeet.uk/csharp/references.html