引用母版页属性

public partial class MasterPage : System.Web.UI.MasterPage
{
    public SqlConnection cnx;
    protected void Page_Load(object sender, EventArgs e)
    {

    }
}

    public SqlConnection CnxInMasterPage
    {
        get { return this.cnx; }
    }

var cnx = ((MasterPage)Master).CnxInMasterPage;

Master

MasterPage

IMyMasterPage

var myMasterPage = this.MasterPage as IMyMasterPage