只要程序一开始执行，每个区域变数就会初始化为所属资料型别的预设值。
Every local variable is initialized to the default value for its data type as soon as the procedure begins running.
此函式用在设定变数的预设值，如果变数的值是空白或尚未设定的话，则会列印出预设值，使用此函式时会预设需要一个参数。
This is used to set a default value for a variable. If the variable is empty or unset, the given default value is printed instead. Default takes one argument.
取得值，指出方法主体中的区域变数是否初始化为其型别的预设值。
Gets a value indicating whether local variables in the method body are initialized to the default values for their types.
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