为指定的全域变数取得值物件。

Gets a value object for the specified global variable.

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类别中的变数栏位与全域或函式变数类似，但它只能在类别范围中作用，而且可以具有控制可视性与用法的不同属性。

A variable field in a class is similar to a global or function variable, except that it is scoped to the class and it can have various attributes governing its visibility and usage.

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编译器最佳化时会排除全域常数，不会为变数保留空间。

The compiler optimizes global constants out, leaving no space reserved for the variable.

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