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