76 lines
1.5 KiB
Markdown
76 lines
1.5 KiB
Markdown
|
---
|
|||
|
|
title: Null-coalescing Operator
|
|||
|
|
localeTitle: 空结合运算符
|
|||
|
|
---
|
|||
|
|
# 空结合运算符
|
|||
|
|
|
|||
|
|
C＃中的null-coalescing运算符用于帮助将一个变量分配给另一个变量，如果源值为`null` ，则指定备用值。 C＃中的null-coalescing运算符是`??` 。
|
|||
|
|
|
|||
|
|
## 例1
|
|||
|
|
|
|||
|
|
由于`name`为`null` ， `clientName`将为`clientName`分配值“John Doe”。
|
|||
|
|
|
|||
|
|
```cs
|
|||
|
|
string name = null;
|
|||
|
|
|
|||
|
|
string clientName = name ?? "John Doe";
|
|||
|
|
|
|||
|
|
Console.WriteLine(clientName);
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
```cs
|
|||
|
|
> John Doe
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
## 例2
|
|||
|
|
|
|||
|
|
由于`name`不为`null` ， `clientName`将为`clientName`分配`name`的值，即“Jane Smith”。
|
|||
|
|
|
|||
|
|
```cs
|
|||
|
|
string name = "Jane Smith";
|
|||
|
|
|
|||
|
|
string clientName = name ?? "John Doe";
|
|||
|
|
|
|||
|
|
Console.WriteLine(clientName);
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
```cs
|
|||
|
|
> Jane Smith
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
## 替代if ... else语句
|
|||
|
|
|
|||
|
|
您可以使用`if...else`语句来测试是否存在`null`并分配不同的值。
|
|||
|
|
|
|||
|
|
```cs
|
|||
|
|
string clientName;
|
|||
|
|
|
|||
|
|
if (name != null)
|
|||
|
|
clientName = name;
|
|||
|
|
else
|
|||
|
|
clientName = "John Doe";
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
但是，使用null-coalescing运算符可以大大简化这一点。
|
|||
|
|
|
|||
|
|
```cs
|
|||
|
|
string clientName = name ?? "John Doe";
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
## 条件（三元）运算符的替代
|
|||
|
|
|
|||
|
|
也可以使用条件运算符来测试`null`的存在并分配不同的值。
|
|||
|
|
|
|||
|
|
```cs
|
|||
|
|
string clientName = name != null ? name : "John Doe";
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
同样，这可以使用null-coalescing运算符进行简化。
|
|||
|
|
|
|||
|
|
```cs
|
|||
|
|
string clientName = name ?? "John Doe";
|
|||
|
|
```
|
|||
|
|
|
|||
|
|
## 参考
|
|||
|
|
|
|||
|
|
* [??运算符（C＃参考）](https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/null-conditional-operator)
|