为什么我的类声明列表在另一个方法中使用时变为空，而不是填充它的方法

I declared my list "lijstBijlagen" at the top of my class so I would be able to use it in all the methods of that class. Now the problem is that I fill this list up in the method "btnUpload_Click" but when I want to use it in the method btnSubmit_Click it becomes empty again. I left out my Page_Load code in the code underneath this text because I don't think you'll need it to solve the problem.I declared my list "lijstBijlagen" at the top o