C#基础-可空类型
转自:http://www.cnblogs.com/yangyancheng/archive/2009/04/29/1446176.html
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
1
#region 可空类型
2
/// <summary>
3
/// 可以为 null 的类型是 System..::.Nullable<(Of <(T>)>) 结构的实例。
4
/// 可以为 null 的类型可以表示其基础值类型正常范围内的值,
5
/// 再加上一个 null 值。例如,Nullable<Int32>
6
/// 读作“可以为 null 的 Int32”,可以将其赋值为 -2147483648 到 2147483647 之间的任意值,
7
/// 也可以将其赋值为 null 值。
8
/// 可以赋给 Nullable<bool> 的值包括 true、false 或 null。
9
/// 在处理数据库和其他包含不可赋值的元素的数据类型时,
10
/// 将 null 赋值给数值类型或布尔型的功能特别有用。
11
/// 例如,数据库中的布尔型字段可以存储值 true 或 false,
12
/// 或者,该字段也可以未定义。
13
/// </summary>
14
public class NullableDemo
15
{
16
public void TestMethod()
17
{
18
///可空类型,可以被赋值为null,也可以正常赋值
19
int? n = 10;
20
n = null;
21
Console.WriteLine(n);
22
///可空类型可以通过HasValue属性判断是否有值
23
if (n.HasValue)
24
{
25
///如果有值,可通过Value属性获得
26
Console.WriteLine(n.Value);
27
}
28
///得到值,如果为空则会得到系统赋的默认值,下面语句输出为0
29
Console.WriteLine( n.GetValueOrDefault());
30![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
31
int m = 10;
32
//下面这行要报错,因为他不是可空类型,不能为null
33
//m = null;
34
///用来接受可空类型与普通值类型计算结果,必须也是可空类型
35
int? result;
36
///以下四行代码均返回null
37
result = m + n;
38
result = m * n;
39
result = m - n;
40
result = m / n;
41
Console.WriteLine(result);
42![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
43
///??空接合操作符
44
///可以用来判断可空类型是否为空
45
///下面这行代码表示如果n!=null 那么x=n,否则 x=-1;
46
int x = n ?? -1;
47
///可以使用表达式,如下示
48
//int y = (n * m) ?? (100 / 2 * 3);
49
Console.WriteLine(x);
50
}
51
}
52
#endregion
53
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
1
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockStart.gif)
2
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
3
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
4
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
5
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
6
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
7
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
8
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
9
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
10
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
11
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
12
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
13
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif)
14
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
15
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
16
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
17
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
18
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
19
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
20
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
21
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
22
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
23
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
24
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
25
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
26
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
27
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif)
28
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
29
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
30
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
31
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
32
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
33
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
34
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
35
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
36
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
37
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
38
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
39
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
40
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
41
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
42
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
43
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
44
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
45
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif)
46
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
47
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockStart.gif)
48
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
49
![](https://www.cnblogs.com/Images/OutliningIndicators/InBlock.gif)
50
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif)
51
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedSubBlockEnd.gif)
52
![](https://www.cnblogs.com/Images/OutliningIndicators/ExpandedBlockEnd.gif)
53
![](https://www.cnblogs.com/Images/OutliningIndicators/None.gif)
可空类型特性:
- 可以为 null 的类型表示可被赋值为 null 值的值类型变量。无法创建基于引用类型的可以为 null 的类型。(引用类型已支持 null 值。)
- 语法 T? 是 Nullable<(Of <(T>)>) 的简写,此处的 T 为值类型。这两种形式可以互换。
- 为可以为 null 的类型赋值的方法与为一般值类型赋值的方法相同,如 int? x = 10; 或 double? d = 4.108;。
- 如果基础类型的值为 null,请使用 Nullable<(Of <(T>)>)..::.GetValueOrDefault 方法返回该基础类型所赋的值或默认值,例如 int j = x.GetValueOrDefault();
- 请使用 HasValue 和 Value 只读属性测试是否为空和检索值,例如 if(x.HasValue) j = x.Value;
- 如果此变量包含值,则 HasValue 属性返回 True;或者,如果此变量的值为空,则返回 False。
- 如果已赋值,则 Value 属性返回该值。否则,将引发 System..::.InvalidOperationException。
- 可空类型变量的默认值将 HasValue 设置为 false。未定义 Value。
- 使用 ?? 运算符分配默认值,当前值为空的可以为 null 的类型被赋值给非空类型时将应用该默认值,如 int? x = null; int y = x ?? -1;。
- 不允许使用嵌套的可以为 null 的类型。将不编译下面一行:Nullable<Nullable<int>> n;