The QueryStringParameter.QueryStringField property identifies a name/value pair that is passed with the query string. The QueryStringParameter.QueryStringField property identifies the name of the pair, whereas the System.Web.UI.WebControls.QueryStringParameter property binds to its corresponding value at run time. If the expected query-string name/value pair is not passed to the page with the query string, the QueryStringParameter.Evaluate(System.Web.HttpContext, System.Web.UI.Control) method then tries to bind the parameter to the value of the Parameter.DefaultValue property. If the Parameter.DefaultValue property is not set, the QueryStringParameter.Evaluate(System.Web.HttpContext, System.Web.UI.Control) method fails to bind the parameter to a value.