如何测试引用同一性（C# 编程指南）
您不必实现任何自定义逻辑来支持类型中的引用相等比较。 静态 Object.ReferenceEquals 方法为所有类型提供此功能。
下面的示例演示如何确定两个变量是否 引用相等性，这意味着它们引用内存中的同一对象。
该示例还显示了为什么 Object.ReferenceEquals 始终返回值类型的 false。 这是由于装箱造成的，此操作会为每个值类型参数创建单独的对象实例。 此外，不应使用 ReferenceEquals 来确定字符串相等性。
例
using System.Text;
namespace TestReferenceEquality
{
struct TestStruct
{
public int Num { get; private set; }
public string Name { get; private set; }
public TestStruct(int i, string s) : this()
{
Num = i;
Name = s;
}
}
class TestClass
{
public int Num { get; set; }
public string? Name { get; set; }
}
class Program
{
static void Main()
{
// Demonstrate reference equality with reference types.
#region ReferenceTypes
// Create two reference type instances that have identical values.
TestClass tcA = new TestClass() { Num = 1, Name = "New TestClass" };
TestClass tcB = new TestClass() { Num = 1, Name = "New TestClass" };
Console.WriteLine("ReferenceEquals(tcA, tcB) = {0}",
Object.ReferenceEquals(tcA, tcB)); // false
// After assignment, tcB and tcA refer to the same object.
// They now have reference equality.
tcB = tcA;
Console.WriteLine("After assignment: ReferenceEquals(tcA, tcB) = {0}",
Object.ReferenceEquals(tcA, tcB)); // true
// Changes made to tcA are reflected in tcB. Therefore, objects
// that have reference equality also have value equality.
tcA.Num = 42;
tcA.Name = "TestClass 42";
Console.WriteLine("tcB.Name = {0} tcB.Num: {1}", tcB.Name, tcB.Num);
#endregion
// Demonstrate that two value type instances never have reference equality.
#region ValueTypes
TestStruct tsC = new TestStruct( 1, "TestStruct 1");
// Value types are boxed into separate objects when passed to ReferenceEquals.
// Even if the same variable is used twice, boxing ensures they are different instances.
TestStruct tsD = tsC;
Console.WriteLine("After assignment: ReferenceEquals(tsC, tsD) = {0}",
Object.ReferenceEquals(tsC, tsD)); // false
#endregion
#region stringRefEquality
// Constant strings within the same assembly are always interned by the runtime.
// This means they are stored in the same location in memory. Therefore,
// the two strings have reference equality although no assignment takes place.
string strA = "Hello world!";
string strB = "Hello world!";
Console.WriteLine("ReferenceEquals(strA, strB) = {0}",
Object.ReferenceEquals(strA, strB)); // true
// After a new string is assigned to strA, strA and strB
// are no longer interned and no longer have reference equality.
strA = "Goodbye world!";
Console.WriteLine("strA = \"{0}\" strB = \"{1}\"", strA, strB);
Console.WriteLine("After strA changes, ReferenceEquals(strA, strB) = {0}",
Object.ReferenceEquals(strA, strB)); // false
// A string that is created at runtime cannot be interned.
StringBuilder sb = new StringBuilder("Hello world!");
string stringC = sb.ToString();
// False:
Console.WriteLine("ReferenceEquals(stringC, strB) = {0}",
Object.ReferenceEquals(stringC, strB));
// The string class overloads the == operator to perform an equality comparison.
Console.WriteLine("stringC == strB = {0}", stringC == strB); // true
#endregion
// Keep the console open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
}
}
/* Output:
ReferenceEquals(tcA, tcB) = False
After assignment: ReferenceEquals(tcA, tcB) = True
tcB.Name = TestClass 42 tcB.Num: 42
After assignment: ReferenceEquals(tsC, tsD) = False
ReferenceEquals(strA, strB) = True
strA = "Goodbye world!" strB = "Hello world!"
After strA changes, ReferenceEquals(strA, strB) = False
ReferenceEquals(stringC, strB) = False
stringC == strB = True
*/
System.Object 通用基类中 Equals 的实现也执行引用相等性检查，但最好不要使用此检查，因为，如果类碰巧重写该方法，结果可能不是预期的结果。 == 和 != 运算符也是如此。 在引用类型上运行时，== 和 != 的默认行为是执行引用相等性检查。 但是，派生类可以重载运算符来执行值相等性检查。 为了最大程度地减少出错的可能性，最好在必须确定两个对象是否具有引用相等性时始终使用 ReferenceEquals。
运行时始终暂存同一程序集内的常量字符串。 也就是说，只维护每个唯一文本字符串的一个实例。 但是，运行时不能保证会暂存在运行时创建的字符串，也不保证会暂存不同程序集中两个相等的常量字符串。
注意
ReferenceEquals因装箱的原因会返回值类型的 false，因为每个参数都会独立装入一个单独的对象中。
