Web API无法隐式转换类型'System.Collections.Generic.List <

Question

I am creating a Web API in I know there are lots of questions asked about it but I have tried their solution but didn't work for me. I am declaring a list which I will be using at several points in my list.

public HttpResponseMessage GetDetails(string msn, DateTime dt)
{
   try
    {
          var mainDetails= new List<string>();

          int mainCount = giveMainCount(msn, dt);

           if(mainCount==0)
           {
              // here I want to set the list empty like mainDetails = null or "" like this
           }

            int mainInterval = mainCount / 500;

            mainDetails = kesc.tj_xhqd
                         .AsNoTracking()
                         .Where(m => (m.zdjh == msn) && (m.sjsj >= dt))
                         .AsEnumerable()
                         .Select((x, i) => new { MSN = x.zdjh, PingDateTime = x.sjsj, PingValue = x.xhqd, i = i })
                         .Where(x => x.i % mainInterval == 0)
                         .ToList(); // here I am getting error 
       return Request.CreateResponse(HttpStatusCode.OK, new { details = mainDetails });

    }
    catch (Exception ex)
    {
        return Request.CreateErrorResponse(HttpStatusCode.NotFound, ex);
    }


    }
}

The error is

Cannot implicitly convert type 'System.Collections.Generic.List<<anonymous type: string MSN, System.DateTime? PingDateTime, string PingValue, int i>>' to 'System.Collections.Generic.List<string>'

How can I get rid of this error ?

Any help would be highly appreciated.

Answer 1

Option 1

You could go with not declaring mainDetails from the beginning, doing something like:

public HttpResponseMessage GetDetails(string msn, DateTime dt)
{
    try
    {
        int mainCount = giveMainCount(msn, dt);

        if (mainCount == 0)
        {
            return Request.CreateResponse(HttpStatusCode.OK, new { details = null });
        }

        int mainInterval = mainCount / 500;

        var mainDetails = kesc.tj_xhqd
                        .AsNoTracking()
                        .Where(m => (m.zdjh == msn) && (m.sjsj >= dt))
                        .AsEnumerable()
                        .Select((x, i) => new { MSN = x.zdjh, PingDateTime = x.sjsj, PingValue = x.xhqd, i = i })
                        .Where(x => x.i % mainInterval == 0)
                        .ToList();

        return Request.CreateResponse(HttpStatusCode.OK, new { details = mainDetails });

    }
    catch (Exception ex)
    {
        return Request.CreateErrorResponse(HttpStatusCode.NotFound, ex);
    }
}

Doing it this way, mainDetails will be what you want, a List of that anonymous type with aliases.

---

Option 2

As an alternative option, you could create a class for the aliases (use what you want instead of AliasClassName ):

public class AliasClassName
{
    public string MSN { get; set; }
    public Nullable<System.DateTime> PingDateTime { get; set; }
    public string PingValue { get; set; }
    public int i { get; set; }
}

Then declare mainDetails as a List of that:

var mainDetails = new List<AliasClassName>();

And then do:

.Select((x, i) => new AliasClassName { MSN = x.zdjh, PingDateTime = x.sjsj, PingValue = x.xhqd, i = i })

Answer 2

I have found a solution and it works for me

Changed

var mainDetails= new List<tj_xhqd>();

To

IEnumerable mainDetails= new List<tj_xhqd>();

By doing this I got rid of my error

For further details, one can check it here