如何将LINQ查询的where子句中的方法/函数调用为IEnumerable对象

本文关键字：方法 函数调用 对象 IEnumerable 子句 LINQ 查询 where | 更新日期: 2023-09-27 18:26:27

我在linq查询中的where子句中使用函数时遇到问题，如下代码所示。

  1. 返回IEnumerable对象的函数：

    private IEnumerable filterparameter(string filter) 
{        
    EmailAflAwmMessageDM obj = new EmailAflAwmMessageDM();
    if (filter == "attachments")
    {
        return obj.attachments;
    }
    else if (filter == "flagged")
    {
        return obj.flagged;
    }
    else
    {
        return obj.seen;
    }
}

  2. 返回IQuerable的函数，该函数将用于web api函数：

    private IQueryable SearchFilterCondition(string filter,string value)
{
    var emailmessage = from a in db.EmailAflAwmMessage
                       where (filterparameter(filter) == value)
                       orderby a.msg_date descending
                       select new
                       {
                           a.subject,
                           a.msg_date,         
                       };
    return emailmessage;
}

更新：此错误

{"Message":"An error has occurred.",
"ExceptionMessage":"The 'ObjectContent`1' type failed to serialize the response body for content type 'application/json; charset=utf-8'.",
"ExceptionType":"System.InvalidOperationException",
"StackTrace":null,
"InnerException": {
    "Message":"An error has occurred.",
    "ExceptionMessage":"LINQ to Entities does not recognize the method 'System.Collections.IEnumerable filterparameter(System.String)' method, and this method cannot be translated into a store expression.",
    "ExceptionType":"System.NotSupportedException",
    "StackTrace":"

请帮我解决这个问题，或者给我找一个替代者。

如何将LINQ查询的where子句中的方法/函数调用为IEnumerable对象

如@Domysee所述，您不能在Linq查询中调用任何c#函数。仅EntitiFunction这就是为什么表达式是为之构建的。要在linq查询中使用

        public Expression<Func<EmailAflAwmMessage, bool>> GetSelectXpr(string filter, string value)
        {
            if (filter == "attachments")
                return e => e.attachments == value;
            else if (filter == "flagged")
                return e => e.flagged == value;
            else
                return e => e.seen == value;
        }

用法。

var selectXpr = GetGetSelectXpr(filter,value);
    var emailmessage = db.EmailAflAwmMessage.Where(selectXpr).OrderByDescending(a=>a.msg_date).Select(a=> 
     new {
     a.subject,
     a.msg_date
   })

不幸的是，不可能在IQueryable的Where中使用方法，因为Linq-to-SQL不知道如何将方法转换为SQL。

您必须将方法内联到Where中才能使其工作
类似的东西：

private IQueryable SearchFilterCondition(string filter,string value)
{
    var filterParam = filterparameter(filter); //evaluate filterparameter outside of the query
    var emailmessage = from a in db.EmailAflAwmMessage
                       where (filterParam == value)
                       orderby a.msg_date descending
                       select new
                       {
                           a.subject,
                           a.msg_date,         
                       };
    return emailmessage;
}
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