在MVC中填充我的模型时出现错误“指定的转换无效”

Question

I have this model

 public class DocumentModel
    {   
        public int documentID { get; set; }
        public String Title { get; set; }
        [DisplayFormat(DataFormatString = "{0:dd/MM/yyyy}")]
        public DateTime DateCreated { get; set; }
        public String Description { get; set; }
        public int authorID { get; set; }
        public String AuthorName { get; set; }
        public int categoryID { get; set; }
        public String Category { get; set; }
        public int topicID { get; set; }
        public String Topic { get; set; }
        [AllowHtml]
        public String DocumentBody { get; set; }
    }

When im about to fill my Model after making a query in mysql here's the code

 result = from DataRow row in data.Rows
                         select new DocumentModel()
                         {
                             documentID = (int)row["document_id"],
                             Title = row["title"].ToString(),
                             DateCreated = (DateTime)row["date_created"],
                             Description = row["description"].ToString(),
                             AuthorName = row["AuthorName"].ToString(),
                             categoryID = (int)row["category"],
                             topicID = (int)row["topicID"]
                         };

Im having an error in the CategoryID and TopicID the error says "Specified cast is not valid." i dont know where did i get that error i already check all the datatypes if its correct and i confirm that its all correct could you give me advice for this? did i miss some codes of wrong implementations?

before i forgot here is my Query code for MYsql

select a.document_id,a.title,date(a.date_created) as date_created,a.Description,c.first_name as AuthorName,ifnull(e.category_ID, 0) as category, ifnull(d.ID,0) as TopicID from tbldocument a
Inner join tbldocumenttree b on a.document_ID = b.Document_ID
left join tblcategory e on b.category_id = e.category_id
Inner join tblauthor c on a.author_id = c.author_id
left join tbldocumenttree d on d.ID = b.parent

Thank you.

Answer 1

This can happen if row["category"] (same applied for row["topicID"]) is NULL in the data row because you can't cast null to non-nullable int. I notice that you are left joining your tblCategory and therefore NULL is a real possibility.

Either fix on the server using a coalescing function (ie to force any NULL category to default to 0) or else check if row["category"]==DBNull.Value before you attempt to cast it, or indeed use Int32.TryParse and have whatever handling for a category of 0, etc.