Unit Test Methods using ViewState Object

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ASP.NET

I have a BasePage class which contains methods to insert/get values from viewstate,session,cache objects.
All the pages in application inherits this BasePage class
i have successfully created unit test for session/cache operation via adapter pattern.
but i am not able to unit test BasePage methods which are using ViewState object.
How can i unit test these methods too.

eg:

public class BasePage:Page
{
    protected void SetValueToViewState(string key,object value)
    {
           this.ViewState.Add(key,value);
     }

   protected object GetValueFromViewState(string key)
  {
          return this.ViewState[key.Trim()];
   }
}