LINQ to Entities无法识别方法'Double Parse（System.String）

Question

我尝试运行报告时收到错误。问题出在这里：model.Referring = Math.Round(_newSurveyResult.Select(m => string.IsNullOrEmpty(m.Question1) ? 0 : Double.Parse(m.Question1)).Average());

public class SummaryDetails

{

    public int ChannelId { get; set; }

    public int ChannelGroupId { get; set; }

    public string Question1 { get; set; }

    public string Question2 { get; set; }

    public string Question3 { get; set; }

    public string Question4 { get; set; }

    public int OrganizationId { get; set; }

}

public ActionResult AreaManager(AreaManagerModel model)

{

    model.ShowCustomerReport = false;

    model.ShowSurveyReport = true;

    LoadModelVariablesonPostBack(model, 8);

    var _newSurveyResult = (

        from ls in SessionHandler.CurrentContext.LennoxSurveyResponses

        join ml in SessionHandler.CurrentContext.MailingListEntries on ls.SurveyCode equals ml.SurveyCode

        join m in SessionHandler.CurrentContext.MailingLists on ml.MailingListId equals m.MailingListId

        join ch in SessionHandler.CurrentContext.Channels on m.ChannelId equals ch.ChannelId

        join cg in SessionHandler.CurrentContext.ChannelGroups on ch.ChannelGroupId equals cg.ChannelGroupId

        join dcg in SessionHandler.CurrentContext.ChannelGroups on cg.ParentChannelGroupId equals dcg.ChannelGroupId

        join ncg in SessionHandler.CurrentContext.ChannelGroups on dcg.ParentChannelGroupId equals ncg.ChannelGroupId

        join pcg in SessionHandler.CurrentContext.ChannelGroups on ncg.ParentChannelGroupId equals pcg.ChannelGroupId

        select new SummaryDetails { 

            OrganizationId = ch.OrganizationId,

            Question1 = ls.Question1Answer,

            Question2 = ls.Question2Answer,

            Question3 = ls.Question3Answer,

            Question4 = ls.Question4Answer,

            ChannelId = ch.ChannelId,

            ChannelGroupId = model.TMId != 0 ? cg.ChannelGroupId : model.DistrictId != 0 ? dcg.ChannelGroupId : model.AreaId != 0 ? ncg.ChannelGroupId :

        }

    );