null（C# 參考） · MSDN C# 編程指南 & 參考手冊 2015

# null（C# 參考） **null** 關鍵字是表示不引用任何對象的 null 引用的文字值。 **null** 是引用類型變量的默認值。普通值類型不能為 null。但是，C# 2.0 引入了可以為 null 值的類型。請參見 [可以為 null 的類型（C# 編程指南）](https://msdn.microsoft.com/zh-cn/library/1t3y8s4s.aspx)。下面的示例演示 null 關鍵字的一些行為： ``` class Program { class MyClass { public void MyMethod() { } } static void Main(string[] args) { // Set a breakpoint here to see that mc = null. // However, the compiler considers it "unassigned." // and generates a compiler error if you try to // use the variable. MyClass mc; // Now the variable can be used, but... mc = null; // ... a method call on a null object raises // a run-time NullReferenceException. // Uncomment the following line to see for yourself. // mc.MyMethod(); // Now mc has a value. mc = new MyClass(); // You can call its method. mc.MyMethod(); // Set mc to null again. The object it referenced // is no longer accessible and can now be garbage-collected. mc = null; // A null string is not the same as an empty string. string s = null; string t = String.Empty; // Logically the same as "" // Equals applied to any null object returns false. bool b = (t.Equals(s)); Console.WriteLine(b); // Equality operator also returns false when one // operand is null. Console.WriteLine("Empty string {0} null string", s == t ? "equals": "does not equal"); // Returns true. Console.WriteLine("null == null is {0}", null == null); // A value type cannot be null // int i = null; // Compiler error! // Use a nullable value type instead: int? i = null; // Keep the console window open in debug mode. System.Console.WriteLine("Press any key to exit."); System.Console.ReadKey(); } } ``` ## C# 語言規范有關詳細信息，請參閱 [C# 語言規范](https://msdn.microsoft.com/zh-cn/library/ms228593.aspx)。該語言規范是 C# 語法和用法的權威資料。 ## 請參閱 [C# 參考](https://msdn.microsoft.com/zh-cn/library/618ayhy6.aspx) [C# 編程指南](https://msdn.microsoft.com/zh-cn/library/67ef8sbd.aspx) [C# 關鍵字](https://msdn.microsoft.com/zh-cn/library/x53a06bb.aspx) [文字關鍵字（C# 參考）](https://msdn.microsoft.com/zh-cn/library/23954zh5.aspx) [默認值表（C# 參考）](https://msdn.microsoft.com/zh-cn/library/83fhsxwc.aspx) [Nothing (Visual Basic)](https://msdn.microsoft.com/zh-cn/library/0x9tb07z.aspx)