在VB.NET中,是否可以将DateTime变量设置为“未设置”?为什么可以将a设置DateTime为Nothing,但不能检查是否为a Nothing?例如: Dim d As DateTime = Nothing Dim boolNotSet As Boolean = d Is Nothing 第二条语句抛出此错误: 'Is' operator does not accept operands of type 'Date'. Operands must be reference or nullable types.