How to Execute Page_Load() in Page's Base Class?

We faced the similar problem, All you need to do is just register the handler in the constructor. :)

public class FactsheetBase : System.Web.UI.Page 
{ 

    public FactsheetBase()
    {
        this.Load += new EventHandler(this.Page_Load);
    }

    public MyPageData Data { get; set; }  
    protected void Page_Load(object sender, EventArgs e) 
    { 
        // get data that‘s common to all implementors of FactsheetBase 
        // and store the values in FactsheetBase‘s properties 
        this.Data = ExtractPageData(Request.QueryString["data"]);             
    } 
}

Another approach would be to override OnLoad() which is less preferred.

public class FactsheetBase : System.Web.UI.Page 
{ 

    public FactsheetBase()
    {
    }

    public MyPageData Data { get; set; }  
    protected override void OnLoad(EventArgs e)
    {
        //your code
        // get data that‘s common to all implementors of FactsheetBase 
        // and store the values in FactsheetBase‘s properties 
        this.Data = ExtractPageData(Request.QueryString["data"]);             

        base.OnLoad(e);
    }
}