How can I remove old values from static class C#?

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WPF

C#4.0

static

Hello everyone,

I want to pass list items to one class to anther class. I have used static keyword. But now, when the application is running still old variable is there.

Actually I have a listview with checkboxes. After checked checkboxes then click a button. Then popup another window page that contain listview with respect to each Parent_File_Name and UserId . I am facing the issue is, after running this windows apps then many time click on the button again both static variable contain whole list items. How can do this? Can you get me?

Please help me...

What I have tried:

In 1st class,

foreach (DocumentsUser item in listView1.SelectedItems)
{
    ImgName.Add(item.Parent_File_Name);
    UserId.Add((int)item.UserId);
}

private static List imgName = new List();
      public static List ImgName { get { return imgName; } }

      private static List userId = new List();
      public static List UserId { get { return userId; } }

2nd class taking those static variables,

foreach (var userId in Qscan.UC_FileMgmt.UserId)  
           {  
               foreach (var imgName in Qscan.UC_FileMgmt.ImgName)  
               {