当我们把引用类型放在?后面时，我们允许实例为. 所以我们有null

• string?[]- 我们允许项目所在的字符串数组null（但数组本身不能null）
• string[]?- 可以是null自身的字符串数组（但不能有null项目）
• string?[]? - 数组及其项目都允许 null

• Pre C#8 it is always `string?[]?`
• Great answer. Some confusion might remain though, I hope this doesn't add to it... Both `Array` and `string` are reference types (which can be null), so `string?` and `string` are equivalent, and `[]` and `[]?` are equivalent (and there is no difference between `string?[]?` and `string[]`) in "vanilla" C# regardless of version. For the distinction to be made, nullable reference types as a feature has to be explicitly activated `#nullable enable` because it is an otherwise breaking change to the language which makes such conventionally equivalent statements non-equivalent.