在将路径传递到文件名之前添加以下行:

Add this line before passing path to file name:

using System.IO;

要检查路径是否存在:

To Check if path exists:

if(!Directory.Exists(path))
            {
//throw exception here or show message box like this:
MessageBox.Show(path);
            }
else
{
//do something here.
}

要检查文件是否存在:

To check if file exists:

if(File.Exists(filepath)
{
//throw exception here or show message box like this:
MessageBox.Show(filepath)
}
else
{
//do something here.
}

只需逐行调试，看看哪里出现此错误

Just debug line by line and see where you are getting this error