如何测试引用相等性（标识）（C# 编程指南）
无需实现任何自定义逻辑，即可支持类型中的引用相等性比较。 此功能由静态 Object.ReferenceEquals 方法向所有类型提供。
以下示例演示如何确定两个变量是否具有引用相等性，即它们引用内存中的同一对象。
该示例还演示 Object.ReferenceEquals 为何始终为值类型返回 false，以及您为何不应使用 ReferenceEquals 来确定字符串相等性。
示例
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 copied on assignment. tsD and tsC have
// the same values but are not the same object.
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
*/
在 System.Object 通用基类中实现 Equals 也会执行引用相等性检查，但最好不要使用这种检查，因为如果恰好某个类替代了此方法，结果可能会出乎意料。 以上情况同样适用于 == 和 != 运算符。 当它们作用于引用类型时，== 和 != 的默认行为是执行引用相等性检查。 但是，派生类可重载运算符，执行值相等性检查。 为了尽量降低错误的可能性，当需要确定两个对象是否具有引用相等性时，最好始终使用 ReferenceEquals。
运行时始终暂存同一程序集内的常量字符串。 也就是说，仅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。