c# – 如何在LINQ查询的where子句中调用方法/函数作为IEnumerabl

发布时间：2020-12-15 23:29:21 所属栏目：百科来源：网络整理

导读：我有问题在 linq查询中的where子句中使用函数,如下面的代码所示. 返回IEnumerable对象的函数： private IEnumerable filterparameter(string filter) { EmailAflAwmMessageDM obj = new EmailAflAwmMessageDM(); if (filter == "attachments") { return obj.

我有问题在 linq查询中的where子句中使用函数,如下面的代码所示.

>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IQuerable的函数,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":"

请帮我解决这个问题或指导我替代.

解决方法

正如@Domysee所提到的,你不能在Linq查询中调用任何c#函数.只有 EntitiFunctions
这就是为什么表达式已经构建的原因.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
   })

（编辑：李大同）

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