对象是引用类型还是值类型？

Question

我对@9​​87654321@仍有疑问。它是任何东西的主要基类，任何类。但它是引用类型还是值类型。或者喜欢这些行为中的哪一个呢？我需要澄清这一点。我很难理解。

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

在这种情况下，它就像一个值类型。如果对象是引用类型，那么为什么 obj2 的值仍然是“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"   

在这种情况下，它的作用类似于引用类型

我们可以推断object 的类型就是你在里面装的东西。它既可以是引用类型，也可以是值类型。这是关于你在里面装的东西。我说的对吗？

Answer 1

对象变量始终是引用类型。 类和字符串是引用类型。结构和枚举是一种值类型。 我从各种资源中整理了一个大例子。

// 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);
    }

参考资料 - http://jonskeet.uk/csharp/references.html

Answer 2

object 变量始终是引用类型。

object 可以通过装箱的力量“引用”一个值类型。该框是一个值的引用类型包装器，object 变量所引用的值。

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

变量o 是对包含x 值的框的引用——但它不是x：

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

这可能对可变值类型更有启发性：

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 是对x 的加框引用，因此更改x 的值对o 没有影响。

将SampleClass 更改为类而不是结构（引用类型而不是值类型）会改变行为：object o = x; 行会使 o 引用与 x 相同的东西，并更改 x 的文本也会改变o的文字。

Answer 3

当你这样做时

obj1 = "NewString";

它实际上持有一个 new 引用，指向另一个内存位置，而不是您之前给 obj2 的位置。当您更改位置obj1 的内容时，您将在obj2 中得到相同的更改。

尝试用

改变obj1的内容

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

您的两个字符串现在都将是 "0ldString"。

这是因为对象是引用类型。

Answer 4

它是一个引用类型

用字符串做一个例子并不是很有启发性，因为字符串也是一个引用类型（很明显SampleClass也是）；您的示例包含零“装箱”。

如果对象是引用类型，那么为什么 obj2 的值仍然是“OldString”

为什么不呢？当您创建 新字符串 时，不会将旧引用更改为指向新字符串。考虑：

 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"