C#提供了一种特殊的数据类型，可为空类型，您可以为其赋值的正常范围以及空值。
如可以在Nullable变量中存储-2，147，483，648到2，147，483，647或NULL之间的任何值，同样，可以在Nullable变量中指定TRUE、FALSE或NULL。声明可为空类型的语法如下所示:-
< data_type> ? <variable_name>=null;
下面的示例演示可为空的数据类型
using System;
namespace CalculatorApplication {
class NullablesAtShow {
static void Main(string[] args) {
int? num1 = null;
int? num2 = 45;
</span><span class="kwd">double</span><span class="pun">?</span><span class="pln"> num3 </span><span class="pun">=</span><span class="pln"> </span><span class="kwd">new</span><span class="pln"> </span><span class="kwd">double</span><span class="pun">?();</span><span class="pln">
</span><span class="kwd">double</span><span class="pun">?</span><span class="pln"> num4 </span><span class="pun">=</span><span class="pln"> </span><span class="lit">3.14157</span><span class="pun">;</span><span class="pln">
</span><span class="kwd">bool</span><span class="pun">?</span><span class="pln"> boolval </span><span class="pun">=</span><span class="pln"> </span><span class="kwd">new</span><span class="pln"> </span><span class="kwd">bool</span><span class="pun">?();</span><span class="pln">
</span><span class="com">//显示值</span><span class="pln">
</span><span class="typ">Console</span><span class="pun">.</span><span class="typ">WriteLine</span><span class="pun">(</span><span class="str">"Nullables at Show: {0}, {1}, {2}, {3}"</span><span class="pun">,</span><span class="pln"> num1</span><span class="pun">,</span><span class="pln"> num2</span><span class="pun">,</span><span class="pln"> num3</span><span class="pun">,</span><span class="pln"> num4</span><span class="pun">);</span><span class="pln">
</span><span class="typ">Console</span><span class="pun">.</span><span class="typ">WriteLine</span><span class="pun">(</span><span class="str">"A Nullable boolean value: {0}"</span><span class="pun">,</span><span class="pln"> boolval</span><span class="pun">);</span><span class="pln">
</span><span class="typ">Console</span><span class="pun">.</span><span class="typ">ReadLine</span><span class="pun">();</span><span class="pln">
</span><span class="pun">}</span><span class="pln">
}
}
编译并执行上述代码时，将生成以下输出-
Nullables at Show: , 45, , 3.14157 A Nullable boolean value:
Null coalescing Operator (?)
Null合并运算符与可为Null的值类型和引用类型一起使用，它用于将操作数转换为另一个可为空值类型操作数的类型，其中可以进行隐式转换。
如果第一个操作数的值为NULL，则运算符返回第二个操作数的值，否则返回第一个操作数的值。
using System;
namespace CalculatorApplication {
class NullablesAtShow {
static void Main(string[] args) {
double? num1 = null;
double? num2 = 3.14157;
double num3;
num3 </span><span class="pun">=</span><span class="pln"> num1 </span><span class="pun">??</span><span class="pln"> </span><span class="lit">5.34</span><span class="pun">;</span><span class="pln">
</span><span class="typ">Console</span><span class="pun">.</span><span class="typ">WriteLine</span><span class="pun">(</span><span class="str">" Value of num3: {0}"</span><span class="pun">,</span><span class="pln"> num3</span><span class="pun">);</span><span class="pln">
num3 </span><span class="pun">=</span><span class="pln"> num2 </span><span class="pun">??</span><span class="pln"> </span><span class="lit">5.34</span><span class="pun">;</span><span class="pln">
</span><span class="typ">Console</span><span class="pun">.</span><span class="typ">WriteLine</span><span class="pun">(</span><span class="str">" Value of num3: {0}"</span><span class="pun">,</span><span class="pln"> num3</span><span class="pun">);</span><span class="pln">
</span><span class="typ">Console</span><span class="pun">.</span><span class="typ">ReadLine</span><span class="pun">();</span><span class="pln">
</span><span class="pun">}</span><span class="pln">
}
}
编译并执行上述代码时，将生成以下输出-
Value of num3: 5.34 Value of num3: 3.14157
