LINQ to Entities 无法识别方法 'Int32 Int32(System.String)' 方法，并且此方法无法转换为存储表达式

Question

I am trying to Query Database Context using Linq to Entities and I am getting this error:

LINQ to Entities does not recognize the method 'Int32 Int32(System.String)' method, and this method cannot be translated into a store expression.`

Code:

public IEnumerable<CourseNames> GetCourseName()
{
   var course = from o in entities.UniversityCourses
                select new CourseNames
                {
                    CourseID = Convert.ToInt32(o.CourseID),
                    CourseName = o.CourseName,
                 };
   return course.ToList();
}

I tried like this after seeing this

public IEnumerable<CourseNames> GetCourseName()
{
    var temp = Convert.ToInt32(o.CourseID);
    var course = from o in entities.UniversityCourses
                 select new CourseNames
                 {
                     CourseID = temp,
                     CourseName = o.CourseName,
                 };
    return course.ToList();
}

But it throws an error:

"The name 'o' does not exist in the current context"

This is my code for the class GetCourseName

namespace IronwoodWeb
{
    public class CourseNames
    {
        public int CourseID { get; set; }
        public string CourseName { get; set; }
    }
}

Answer 1

public IEnumerable<CourseNames> GetCourseName()
{
    var courses = from o in entities.UniversityCourses
                  select new { o.CourseID, o.CourseName };

    return courses.ToList() // now we have in-memory query
                  .Select(c => new CourseNames()
                  {
                     CourseID = Convert.ToInt32(c.CourseID), // OK
                     CourseName = c.CourseName
                  });
}

Answer 2

If you dont want to materialize the query (retrieve the data) you can use cast (ie (int) o.CourseId). Is converted to SQL CAST AS statement.

Answer 3

You could also bring back the value as a string (as it is apparently stored) and then Convert it after.

The error on 'o' being out of the context is that you are only declaring o in the Linq query and it can only be referenced in that scope.

Answer 4

Explicit conversion is simple and works: (int)o.CourseID

var course = from o in entities.UniversityCourses
                select new CourseNames
                {
                    CourseID = (int)o.CourseID,
                    CourseName = o.CourseName,
                 };