C# - Unable To Understand "A Field Initializer Cannot Reference The Nonstatic Field" Error?

I am getting the error "A field initializer cannot reference the nonstatic field", While My code is as below:

Object selectedItem = PageVariables.slectedItemData;
MyClass selectedItems = (MyClass)selectedItem;

But the same thing works if assign the value at the constructor or in a different method , like below:

public partial class MusicPlayer : Page
{
Object selectedItem = PageVariables.slectedItemData;
[code]...

I am just trying to understand what is the difference, Why it is looking for a static varaible declaration(in 1st case) while doesn't look for it while in a constructor or a different method