Store Class in ViewState

Yes . You can store class object in Page view state.

Let assume, we have a class "ABC"

Class ABC
{
   public string a{get;set;}
   public string b{get;set;}

}

Now we want to store this class on view state on page load

protected void Page_Load(object sender, EventArgs e)
       {
           if (!IsPostBack)
           {
          ABC objAbc = new ABC();
           ViewState["CheckPoint"] = objAbc ;
       }
     }

You can access this object on any other event or other function of the page.

protected void Button1_Click(object sender, EventArgs e)
       {
           ABC objAbc = = (ABC)ViewState["CheckPoint"];
       }

You can keep object of a class in a viewstate, and later you can cast back class object from viewstate into a class object.

SampleClass classObj = new SampleClass();
ViewState["test"] = classObj;
SampleClass obj = (SampleClass)classObj;

Yes you can store a class in the ViewState, just make sure the class is marked as [Serializable] otherwise you may encounter some errors.

[Serializable]
public class MyClass
{
   public string StringProperty { get; set; }
}

public MyClass myClass
{
   get { return (MyClass)ViewState["MyClass"]; }
   set { ViewState["MyClass]" = value; }
}

Don't forget the cast on the Return of the property.

Yes you can store Id in viewstate, but class viewstate make no sense
By storing Id you can call this only that particular page beacuse the viewstate expires with single page in which if you will use viewstate