我该怎么做.
您需要首先在该文件夹中找到所有文件，然后循环/查找所需的文件.如果一次只有一个文件，请使用该文件.

要查找文件夹中的所有文件，请执行以下操作:从目录[C#]中获取文件 [ ^ ]

How can i do that.
You need to find all the files in that folder first and then loop/find the one that you need to. If there would be only one file at a time, then use that one.

To find all files in a folder: Get Files from Directory [C#][^]