用户代码未处理实体异常

Question

I am getting an error called "entity exception was unhandled by user code" in the foreach loop. Why is this happening? What am i doing wrong?

    public IList<ProductDTO> GetProducts()
    {
        IList<ProductDTO> listofproducts = new List<ProductDTO>();
        using (var db = new NORTHWNDEntities())
        {
            var query = from p in db.Products
                        select new
                                   {
                                       Name = p.ProductName,
                                   };

The error happens right her in the foreach.

                *foreach (var product in query)*
                {
                    listofproducts.Add(new ProductDTO { Name = product.Name });
                }
            }
        return listofproducts;

    }

Answer 1

If you want to find the exact problem where is the problem please write code in try catch statement in this way

try
{
  //write code

} 
catch (System.Data.Entity.Validation.DbEntityValidationException ex)
            {
                var outputLines = new List<string>();
                foreach (var eve in ex.EntityValidationErrors)
                {
                    outputLines.Add(string.Format(
                        "{0}: Entity of type \"{1}\" in state \"{2}\" has the following validation errors:",
                        DateTime.Now, eve.Entry.Entity.GetType().Name, eve.Entry.State));
                    foreach (var ve in eve.ValidationErrors)
                    {
                        outputLines.Add(string.Format(
                            "- Property: \"{0}\", Error: \"{1}\"",
                            ve.PropertyName, ve.ErrorMessage));
                    }
                }
                System.IO.File.AppendAllLines(@"c:\temp\errors.txt", outputLines);
            }

Answer 2

First of all check if you can access data source. If it is fine then you need to check your query structure. You are iterating through the query but not through query results. Use ToList method to convert the query to List to iterate through it. You need to to use Enumerable.ToList method to convert the query results to list.

var query = (from p in db.Products
              select new
              {
                    Name = p.ProductName,
              }).ToList();

foreach (var product in query)*
{
      listofproducts.Add(new ProductDTO { Name = product.Name });
}

You can directly create object of ProductDTO using projection.

IList<ProductDTO> listOfProcuts = (from p in db.Products
                                   select new ProductDTO 
                                   {
                                       Name = p.ProductName,
                                   }).ToList();

return listOfProcuts ;

Answer 3

Adil's answer is correct and would solve your problem, but you might consider returning IEnumerable<ProductDTO> instead of IList<DTO> .

Following the principle of information hiding, if the other parts of the program that call GetProducts() do not need list semantics then they shouldn't see the IList<T> semantics