GroupBy in LINQ

Question

I need to group the table according to the Month in the table.

The query goes this way:

var query =
    from a in db.Dates
    from b in db.Facts
    where a.Count_Key == b.Date_key
    select new
    {
        a.Month,
        b.Fact_key
    };

From this query I try to group by Month

query = query.GroupBy(x => x.Month);
Grid1.DataSource = query;
Grid1.DataBind();

Then I get the following error which says:

Cannot implicitly convert IGrouping int ? into IQueryable

Answer 1

You can't reassign back to the query variable as the results of those two queries are different types.

Your first query is essentially this:

IQueryable<'a> query =
    from a in db.Dates
    from b in db.Facts
    where a.Count_Key == b.Date_key
    select new
    {
        a.Month,
        b.Fact_key
    };

Ie it returns a straight IQueryable of the anonymous type.

Your second query returns an IQueryable<IGrouping<'a>> , ie an IQueryable of a group of the anonymous type:

IQueryable<IGrouping<'a>> groupedQuery = query.GroupBy(x => x.Month);

Therefore because the return types are different you can't assign the result of the grouping back to the original variable.

Answer 2

You can do this:

var query2 = query.GroupBy(x => x.Month);
Grid1.DataSource = query2;
Grid1.DataBind();

The problem is caused because the var query is implicitly inferred from its usage. The query is of type IQueryable<SomeAnonymousType> and the GroupBy method returns an IQueryable<IGrouping<int?, SomeAnonymousType>> . Those are simply two different types.