HOW TO：參考相等 (識別) 的測試 (C# 程式設計手冊)
您不需實作任何自訂邏輯，即可支援型別中的參考相等比較。 這個功能是透過靜態 ObjectReferenceEquals() 方法，提供給所有型別。
下列範例顯示如何判斷兩個變數是否具有「參考相等」(Reference Equality)，這代表兩個變數參考記憶體中的相同物件。
範例同時顯示為何 ObjectReferenceEquals() 一律對實值型別傳回 false，以及為何不應使用 ReferenceEquals() 來判斷字串相等。
範例
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 asignment: 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 asignment: 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 asignment: ReferenceEquals(tcA, tcB) = True
tcB.Name = TestClass 42 tcB.Num: 42
After asignment: ReferenceEquals(tsC, tsD) = False
ReferenceEquals(strA, strB) = True
strA = "Goodbye world!" strB = "Hello world!"
After strA changes, ReferenceEquals(strA, strB) = False
*/
在 System.Object 通用基底類別中實作 Equals 也會執行參考相等檢查，但最好不要使用這個用法，因為如果類別覆寫了方法，產生的結果可能不是您想要的。 對 == 和 != 運算子來說，這同樣是成立的。 == 和 != 在參考型別上操作時，其預設行為是執行參考相等檢查。 不過，衍生類別也可以多載運算子來執行實值相等檢查。 為了降低發生錯誤的可能，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()。
相同組件中的常數字串一律由執行階段暫留。 也就是說，每個唯一的常值字串只會保留一個執行個體。 不過，執行階段不保證一定暫留執行階段建立的字串，也不保證會暫留不同組件中的兩個相等常數字串。