C＃-LINQ查询向表返回null并引发异常

Question

I have a piece of code:

var tblGroupedMultiPassive = dtCSV.AsEnumerable()
                                  .Where(r => r.Field<String>("course_type_id") == "3")
                                  .GroupBy(r => new
                                  {
                                       product_id = r.Field<String>("product_id"),
                                       owner_org_id = r.Field<String>("owner_org_id"),
                                  });

if (tblGroupedMultiPassive.Count() > 0)
    dtCSVMultiSCOPassive = tblGroupedMultiPassive.Where(grp => grp.Count() > 1)
                                                 .SelectMany(grp => grp)
                                                 .CopyToDataTable();

Basically on the final statement where assigning to dtCSVMultiSCOPassive, it throws an exception for there being no rows. I know there are rows before this query so it's got to be the LINQ query itself eliminating all of the rows. This is fine, but I need to be able to deal with this situation without it becoming an exception. Any ideas?

Answer 1

You might need to break this into two statements:

DataTable dtCSVMultiSCOPassive = new DataTable();

var query = tblGroupedMultiPassive.Where(grp => grp.Count() > 1).SelectMany(grp => grp);

if(query.Any())
{
    dtCSVMultiSCOPassive = query.CopyToDataTable();
}

Answer 2

I would appear that grp.Count() is always 1, which would indicate that you have unique combinations of product_id and owner_org_id in your first query.

BTW, I believe that .SelectMany(grp => grp) is completely redundant.

Answer 3

Are you sure that this statement doesn't return null:

dtCSVMultiSCOPassive = tblGroupedMultiPassive.Where(grp => grp.Count() > 1);

?

So because of the first conditional tblGroupedMultiPassive.Count() > 0 seems to be true I would try:

if (tblGroupedMultiPassive.Count() > 0)
{
    dtCSVMultiSCOPassive = tblGroupedMultiPassive.Where(grp => grp.Count() > 1);

    if(dtCSVMultiSCOPassive != null)
       dtCSVMultiSCOPassive = dtCSVMultiSCOPassive.CopyToDataTable();
}

Infact the problem could be that almost every grp contains only one element so the query returns null becuase of the second conditional grp.Count() > 1 in your query.