如何測試參考相等 (身分識別) (C# 程式設計手冊)

不必實作任何自訂邏輯，就能支援您類型中的參考相等比較。 此功能是透過靜態 Object.ReferenceEquals 方法提供給所有類型。

下列範例示範如何判斷兩個變數是否具有「參考相等」，這表示它們會參考記憶體中的相同物件。

範例中也會示範為何 Object.ReferenceEquals 一律對值類型傳回 false，以及為何不應使用 ReferenceEquals 來判斷字串是否相等。

範例

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
*/

實作 System.Object 通用基底類別中的 Equals 也會執行參考相等檢查，但最好不要使用此功能，原因是如果類別恰好覆寫方法，結果可能不如預期。 == 和 != 運算子也同樣如此。 當它們對參考型別進行操作時，== 和 != 的預設行為是執行參考相等檢查。 然而，衍生的類別可以多載運算子，以執行值相等檢查。 為了將錯誤的可能性降到最低，建議您在需要判斷兩個物件是否具有參考相等時，最好一律使用 ReferenceEquals。

相同的組件中的常數字串一律由執行階段暫留。 也就是說，會維護每個唯一的常值字串只有一個執行個體。 不過，執行階段不保證暫留在執行階段建立的字串，也不保證暫留在不同組件中的兩個相等常數字串。

另請參閱

• 相等比較