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语法：IsNothing(<表达式>)
说明：如果表达式是一个空引用指针对象，返回True。
示例：{IsNothing(MiddleInitial)}
比如，我们可以使用 IsNothing 函数来判断报表是否有参数值输入，然后配合IIF函数来实现如下效果：
如果未输入参数，报表内容则显示“未输入参数值”
如果输入了参数，报表内容则显示出参数的值。
1. 新建报表，并为报表定义一个可忽略的参数p1。
2. 在报表中添加一个文本框组件，并为文本框内添加表达式 {IIF(IsNothing(@p1), "未输入参数值", @p1)}。
3. 预览报表，查看效果。默认不需要输入参数，报表内容呈现“ 未输入参数值 ”；当输入参数值时，报表内容呈现参数的数值。
