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

C#

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) = {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) = {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 = {tcB.Name} tcB.Num: {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) = {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) = {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 = '{strA}' strB = '{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) = {Object.ReferenceEquals(stringC, strB)}");

            // The string class overloads the == operator to perform an equality comparison.
            Console.WriteLine($"stringC == strB = {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
*/

Equals 通用基类中 System.Object 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。

备注

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。

另请参阅

• 相等性比较