当我尝试对DateTime进行排序时，为什么我的Linq查询会抛出异常？

Question

I have this method that is meant to filter my blog entries by month...

public IList<KeyValuePair<string, int>> GetBlogsByMonth(PteDotNetCore.EnumsAndConstants.BlogType blogTypeArg)
        {
            var blogsOfType = from b in dataContext.blogs
                               where b.BlogType == (int)blogTypeArg
                               select b;


            IList<KeyValuePair<string, int>> retVal = new List<KeyValuePair<string, int>>();

            for(int i= 12; i > 1; i--)
            {
                var blogsInMonth = from b in blogsOfType
                                   where ((DateTime)b.DateAdded).Month == DateTime.Now.AddMonths(-i).Month
                                   select b;

                KeyValuePair<string, int> keyToAdd = new KeyValuePair<string, int>(DateTime.Now.AddMonths(-i).Month.ToString(), blogsInMonth.ToList().Count);

                retVal.Add(keyToAdd);

            }

            return retVal;

        }

When I use the debugger blogsInMonth I get an error

base {System.SystemException} = {"Nullable object must have a value."}

Blog is an entity framework object. To be honest Im a bit confused as to why I have to look in the Value of the blog.DateTime object. Even when I change it to...

var blogsInMonth = from b in blogsOfType
                                   where ((DateTime)b.DateAdded).Month == DateTime.Now.AddMonths(-i).Month
                                   select b;

It still doesnt work...Hope someone can help...

Answer 1

Most likely the Blog.DateAdded column is nullable in the database and there exists null values in the column.

You can try

where b.DateAdded != null && ((DateTime)b.DateAdded).Month == DateTime.Now.AddMonths(-i).Month

Answer 2

I think this is because your b.DateAdded is null. Try changing the lines at the top as follows:

var blogsOfType = from b in dataContext.blogs
    where b.DateAdded != null && b.BlogType == (int)blogTypeArg
    select b;

As a matter of optimization, you can calculate "i moths back" outside of your Linq expression. In case there are many blog entries, you wouldn't be doing DateTime.Now.AddMonth(-1) more times than necessary.