LINQ to Entities中仅支持无参数构造函数和初始值设定项.(ASP.NET Web API)

Question

我有WebAPI控制器从表数据生成XML

这是代码

  public class XMLController : ApiController
{
    private trackingappEntities db = new trackingappEntities();
    [HttpGet]
    public HttpResponseMessage Index()
    {
        var xdoc = new XDocument(
            new XElement("data",
                db.TimeTables.Select(w =>
                    new XElement("worker",
                        new XAttribute("id", w.INN),
                        new XElement("start", w.StartDay),
                        new XElement("pause", w.StartPause),
                        new XElement("continue", w.EndPause),
                        new XElement("end", w.EndDay)
                    )
                )
            )
        );

        return new HttpResponseMessage() { Content = new StringContent(xdoc.ToString(), Encoding.UTF8, "application/xml") };
    }
}

这是TimeTable类

 public partial class TimeTable
{
    public int Id { get; set; }
    public string Company { get; set; }
    public string INN { get; set; }
    public string StartDay { get; set; }
    public string StartPause { get; set; }
    public string EndDay { get; set; }
    public string EndPause { get; set; }
}

但是当我发送请求时,我有这个回复

LINQ to Entities中仅支持无参数构造函数和初始值设定项.

我怎么解决这个问题？

Answer 1

LINQ的工作原理是检查函数调用的代码,而不仅仅是结果.

在您的情况下,可能的解决方案是AsEnumerable在调用构造函数之前调用告诉LINQ从SQL填充每个记录.

var timeTables = db.TimeTables
    /*
     * Select is optional; this would improve performance for a table with many 
     * extra properties, but would have little-to-no relevant impacts if you're
     * already using them all. Similar to a SELECT * versus selecting individual
     * columns.
     */
    .Select(c => new {
        c.INN,
        c.StartDay,
        c.StartPause,
        c.EndPause,
        c.EndDay
    })
    .AsEnumerable(); // or ...await ToListAsync();

var xdoc = new XDocument(
        new XElement("data",
            timeTables.Select(w =>
                new XElement("worker",
                    new XAttribute("id", w.INN),
                    new XElement("start", w.StartDay),
                    new XElement("pause", w.StartPause),
                    new XElement("continue", w.EndPause),
                    new XElement("end", w.EndDay)
                )
            )
        )
    );

否则,您可以想象LINQ尝试基本上发送逻辑以XElement直接生成SQL.由于SQL无法理解该过程中涉及的逻辑,因此LINQ抛出.

在此示例中,Select调用构建匿名类型.因为它保证在构造函数中没有执行额外的逻辑,所以LINQ能够简单地选择这些值,并将它们填充到C#-side.