当尝试从列表对象传递列值时，Linq查询返回null

问题描述

当尝试传递从列表对象获取的列值时，Linq查询返回null。是否可以做在代码中做。期望的答案或一些建议。

Linq query returning null when trying to pass a column value fetched from list objects . Is it possible to do as done in the code. Expecting answer or some suggestion.

var query = from p in context.ProcessStepTables
            where (p.DiagramID == diagramInfo.DiagramID)
            orderby p.ProcessNo select new{
                  DiagramProcessID = p.DiagramProcessID,
                  ProcessNo = p.ProcessNo,
                  ProcessID = p.ProcessID,
                  ProcessName = Process().Find(x => 
                           p.ProcessID == x.ProcessID).ProcessName.ToString(),
                  MakerName = Maker().Find(x=>
                           p.MakerID==x.MakerID).MakerName.ToString(),
                  Price = p.Price,
                  Note = p.Note,
                  Notice = p.Notice
            };

private List<MakerTable> Maker()
{
   List<MakerTable> pList = new List<MakerTable>();
   try
   {
       IQueryable<MakerTable> maker = (from data in context.MakerTables
                                       select data) as IQueryable<MakerTable>;
       foreach (MakerTable val in maker)
       {
          pList.Add(val);
       }
       return pList.OrderBy(x => x.MakerName).ToList();

    }
    catch (Exception ex)
    {
       MessageBox.Show(ex.Message);
       return null;
    }
}

推荐答案

是因为，你的提供者不知道 .ToString（）方法，即，当你创建一个IQueryable形式的查询，它被翻译成等价的SQL查询，所以如果你包括任何C＃函数，事件一个非原始数据类型，它会抛出你的错误，因为你的查询被构造如下：

that is because, your provider doesn't know about .ToString() method i.e., when you create a query in IQueryable form, it is translated into equivalent SQL query, so if you include any C# function, event a non-primitive datatype, it will throw you that error, because your query gets constructed something like below:

"Select s.DiagramProcessID as DiagramProcessID, ...other fields..
 from MakerTables s where something.ToString()=='anyvalue'"

很明显，sql不知道任何关于 .ToString（）。

so obviously, sql doesn't know anything about .ToString().

简单的方法，避免是，在应用 .ToList（）到您的查询后，执行您的自定义选择。

Simply way to avoid is to, perform your custom Select, after applying .ToList() to your query.

当你这样做或 .AsEnumerable（）时，查询在数据库上执行，现在无论什么自定义选择或where子句，在CLR上

When you do that or .AsEnumerable(), query is executed on the database and now whatever custom selection or where clause is there, is translated on the CLR

请尝试：

var query = context.ProcessStepTables
            .Where(s=>s.DiagramID == diagramInfo.DiagramID)
            .OrderBy(s=>s.ProcessNo)
            .ToList() //this will cause the query to be executed on the db
             //Now perform the selection on returned result set, now the linq
             //has to do with this dataset
            .Select(s=>new
             {
                   DiagramProcessID = s.DiagramProcessID,
                   ProcessNo = s.ProcessNo,
                   ProcessID = s.ProcessID,
                   //other items in your custom list
             });

，您也可以用以下替换您的Maker方法：

and also you can replace your Maker method with below:

private List<MakerTable> Maker()
{
    try
    {
       return context.MakerTables.OrderBy(x=>x.MakerName).ToList();
    }
    catch (Exception ex)
    {
       MessageBox.Show(ex.Message);
       return null;
    }
}

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