如何測試參考相等 （身分識別） （C# 程式設計手冊）

您不需要實作任何自定義邏輯，即可支持類型中的參考相等比較。 靜態 Object.ReferenceEquals 方法為所有類型提供這項功能。

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

此範例還示範了為什麼 Object.ReferenceEquals 總是傳回實值類型的 false。 這是因為 boxing，這會為每個值類型參數建立個別的物件實例。 此外，您不應該使用 ReferenceEquals 來判斷字串相等。

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) = {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
*/

System.Object 通用基類中的 Equals 實作也會執行參考相等檢查，但最好不要使用此檢查，因為，如果類別碰巧覆寫 方法，結果可能不是您預期的結果。 == 和 != 運算符也是如此。 在參考型別上運作時，== 和 != 的預設行為是執行參考相等檢查。 不過，衍生類別可以多載 運算符來執行值相等檢查。 若要將錯誤的可能性降到最低，當您必須判斷兩個物件是否具有參考相等時，最好一律使用 ReferenceEquals。

相同元件內的常數字串一律由執行時期內部儲存。 也就是說，只會維護每個唯一常值字串的一個實例。 執行階段不保證在執行期間建立的字串會被實體化，也不保證不同組件中的兩個相等的常數字串會被實體化。

注意

ReferenceEquals 因為 將進行 boxing 而傳回值型別的 false，因為每個參數都會獨立封箱成為個別的物件。

另請參閱

• 相等比較