如何在LINQ查询的where子句中将方法/函数作为IEnumerable对象调用

我where在linq查询的子句中使用函数时遇到问题，如以下代码所示。

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;
    }
}

返回的函数IQuerable，将在Web api函数中使用：

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":"

请帮助我解决此问题或指导我。

正如提到的@Domysee所述，您不能在Linq查询中调用任何c＃函数。仅EntitiFunctions 这就是为什么要为其构建表达式的原因。在linq查询中使用

        public Expression> 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
   })