其實質上是在Framework中增加了一個Nullable<T>的泛型結構類型。
[SerializableAttribute()] public struct Nullable<T> : IFormattable, IComparable, INullableValue where T : ValueType
看看上面的聲明，我們可以確定，Nullable是一個值類型，而且限制類型參數為值類型。
另外C#在語法層面作了簡化，見下面的代碼。
int? x = 3; Nullable<int> y = new Nullable<int>(3);
看看反編譯的結果。
Nullable<int> nullable1 = new Nullable<int>(3); Nullable<int> nullable2 = new Nullable<int>(3);
實際上，編譯器會將縮寫方式處理成完整的結構體創建代碼。
Nullable包含兩個有用的屬性，HasValue用來判斷類型是否為空，如果不為空則可以通過Value屬性獲取其基礎類型的值。
當其值不為空時，可以直接使用其基礎類型的運算符進行操作。GetValueOrDefault 方法可以獲取值或者基礎類型的缺省值。
int? x = 3; int y = 1; Console.WriteLine(x + y);
不過看看這段代碼的反編譯結果，你可能會發現一些問題。
Nullable<int> nullable1 = new Nullable<int>(3); int num1 = 1; Nullable<int> nullable2 = nullable1; int num2 = num1; Console.WriteLine( nullable2.get_HasValue() ? new Nullable<int>(nullable2.GetValueOrDefault() + num2) : new Nullable<int>());
原本簡單的代碼變得很復雜，編譯器創建了新的Nullable對象，而且通過判斷，如發現Nullable對象為空，則放棄加法操作，直接返回空。
繼續看反編譯的IL代碼，還出現了box指令，因此Nullable類型的代價是很高的，如非必須，不要使用。
.entrypoint // Code Size: 63 byte(s) .maxstack 3 .locals init ( [mscorlib]System.Nullable`1<int32> nullable1, int32 num1, [mscorlib]System.Nullable`1<int32> nullable2, int32 num2, [mscorlib]System.Nullable`1<int32> nullable3) L_0000: nop L_0001: ldloca.s nullable1 L_0003: ldc.i4.3 L_0004: call instance void [mscorlib]System.Nullable`1<int32>::.ctor(!0) L_0009: nop L_000a: ldc.i4.1 L_000b: stloc.1 L_000c: ldloc.0 L_000d: stloc.2 L_000e: ldloc.1 L_000f: stloc.3 L_0010: ldloca.s nullable2 L_0012: call instance bool [mscorlib]System.Nullable`1<int32>::get_HasValue() L_0017: brtrue.s L_0025 L_0019: ldloca.s nullable3 L_001b: initobj [mscorlib]System.Nullable`1<int32> L_0021: ldloc.s nullable3 L_0023: br.s L_0033 L_0025: ldloca.s nullable2 L_0027: call instance !0 [mscorlib]System.Nullable`1<int32>::GetValueOrDefault() L_002c: ldloc.3 L_002d: add L_002e: newobj instance void [mscorlib]System.Nullable`1<int32>::.ctor(!0) L_0033: box [mscorlib]System.Nullable`1<int32> L_0038: call void [mscorlib]System.Console::WriteLine(object) L_003d: nop L_003e: ret
最后提一下 ?? 這個操作符。
The ?? defines a default value that is returned when a nullable type is assigned to a non-nullable type.
// ?? operator example. int? x = null; int y = x ?? -1; // Assigns y to -1 if x is null.
Nullable<T> Struct
Represents a value type that can be assigned null.
public struct Nullable<T> where T : struct- T
The underlying value type of the Nullable<T> generic type.
- Inheritance
Examples
The following code example defines three rows of a table in the Microsoft Pubs sample database. The table contains two columns that are not nullable and two columns that are nullable.
using System; class Sample { // Define the "titleAuthor" table of the Microsoft "pubs" database. public struct titleAuthor { // Author ID; format ###-##-#### public string au_id; // Title ID; format AA#### public string title_id; // Author ORD is nullable. public short? au_ord; // Royalty Percent is nullable. public int? royaltyper; } public static void Main() { // Declare and initialize the titleAuthor array. titleAuthor[] ta = new titleAuthor[3]; ta[0].au_id = "712-32-1176"; ta[0].title_id = "PS3333"; ta[0].au_ord = 1; ta[0].royaltyper = 100; ta[1].au_id = "213-46-8915"; ta[1].title_id = "BU1032"; ta[1].au_ord = null; ta[1].royaltyper = null; ta[2].au_id = "672-71-3249"; ta[2].title_id = "TC7777"; ta[2].au_ord = null; ta[2].royaltyper = 40; // Display the values of the titleAuthor array elements, and // display a legend. Display("Title Authors Table", ta); Console.WriteLine("Legend:"); Console.WriteLine("An Author ORD of -1 means no value is defined."); Console.WriteLine("A Royalty % of 0 means no value is defined."); } // Display the values of the titleAuthor array elements. public static void Display(string dspTitle, titleAuthor[] dspAllTitleAuthors) { Console.WriteLine("*** {0} ***", dspTitle); foreach (titleAuthor dspTA in dspAllTitleAuthors) { Console.WriteLine("Author ID ... {0}", dspTA.au_id); Console.WriteLine("Title ID .... {0}", dspTA.title_id); Console.WriteLine("Author ORD .. {0}", dspTA.au_ord ?? -1); Console.WriteLine("Royalty % ... {0}", dspTA.royaltyper ?? 0); Console.WriteLine(); } } } // The example displays the following output: // *** Title Authors Table *** // Author ID ... 712-32-1176 // Title ID .... PS3333 // Author ORD .. 1 // Royalty % ... 100 // // Author ID ... 213-46-8915 // Title ID .... BU1032 // Author ORD .. -1 // Royalty % ... 0 // // Author ID ... 672-71-3249 // Title ID .... TC7777 // Author ORD .. -1 // Royalty % ... 40 // // Legend: // An Author ORD of -1 means no value is defined. // A Royalty % of 0 means no value is defined. Remarks
A type is said to be nullable if it can be assigned a value or can be assigned null, which means the type has no value whatsoever. By default, all reference types, such as String, are nullable, but all value types, such as Int32, are not.
In C# and Visual Basic, you mark a value type as nullable by using the ? notation after the value type. For example, int? in C# or Integer? in Visual Basic declares an integer value type that can be assigned null.
The Nullable<T> structure supports using only a value type as a nullable type because reference types are nullable by design.
The Nullable class provides complementary support for the Nullable<T> structure. The Nullable class supports obtaining the underlying type of a nullable type, and comparison and equality operations on pairs of nullable types whose underlying value type does not support generic comparison and equality operations.
Fundamental Properties
The two fundamental members of the Nullable<T> structure are the HasValue and Value properties. If the HasValue property for a Nullable<T> object is true, the value of the object can be accessed with the Value property. If the HasValue property is false, the value of the object is undefined and an attempt to access the Value property throws an InvalidOperationException.
Boxing and Unboxing
When a nullable type is boxed, the common language runtime automatically boxes the underlying value of the Nullable<T> object, not the Nullable<T> object itself. That is, if the HasValue property is true, the contents of the Value property is boxed. When the underlying value of a nullable type is unboxed, the common language runtime creates a new Nullable<T> structure initialized to the underlying value.
If the HasValue property of a nullable type is false, the result of a boxing operation is null. Consequently, if a boxed nullable type is passed to a method that expects an object argument, that method must be prepared to handle the case where the argument is null. When null is unboxed into a nullable type, the common language runtime creates a new Nullable<T> structure and initializes its HasValue property to false.
.NET Framework 4.5.1 and Windows Runtime Components
Starting with the .NET Framework 4.5.1, you can include a Nullable<T> type as a member of a structure exported in a WinMD library. Previously, this was not supported.
Constructors
| Nullable<T>(T) | Initializes a new instance of the Nullable<T> structure to the specified value. |
Properties
| HasValue | Gets a value indicating whether the current Nullable<T> object has a valid value of its underlying type. |
| Value | Gets the value of the current Nullable<T> object if it has been assigned a valid underlying value. |
Methods
| Equals(Object) | Indicates whether the current Nullable<T> object is equal to a specified object. |
| GetHashCode() | Retrieves the hash code of the object returned by the Value property. |
| GetValueOrDefault() | Retrieves the value of the current Nullable<T> object, or the object's default value. |
| GetValueOrDefault(T) | Retrieves the value of the current Nullable<T> object, or the specified default value. |
| ToString() | Returns the text representation of the value of the current Nullable<T> object. |
Operators
| Explicit(Nullable<T> to T) | |
| Implicit(T to Nullable<T>) |