The last set includes a colon (:) between the variable name and the substitution operator, which adds the check to see if the variable is NULL, as well.

后一组定义在变量名和替换操作符之间包含一个冒号(：)，这会额外检查变量是否是NULL。

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Nullable types and null coalescing operator

可置空类型和空合并运算符

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Nullable types and null coalescing operator

可置空类型和空合并运算符

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