Linq to DataTable - Cannot cast DBNull

问题:

New to Linq, so apologies if this is basic. This query is throwing up the error {"Cannot cast DBNull.Value to type 'System.Int64'. Please use a nullable type."} when I enumerate the results.

    private void AddLevels(long rootid)
    {
        var results = from row in data.AsEnumerable()
                      where row.Field<long>("ParentID") == rootid
                      select row;

        foreach (DataRow row in results)
        {
            //do stuff
        }

    }

The column ParentID does accept nulls - do I need to handle these separately?

EDIT2: Actual solution below that still uses Linq.

EDIT: I solved this by scrapping Linq and just using a DataTable.Select statement instead. If anyone's got input on the performance difference I'd be interested.

回答1:

Use this line in your query:

where row.Field<decimal?>("ParentID") == rootid

decimal? is syntactic sugar for System.Nullable<decimal>, which is essentially the same as decimal, except that it also allows for null values.

long is a different type altogether -- it can only represent integers and not decimal values, hence the "Specified cast is not valid" error.

回答2:

long rootid is nullable type? It should be then only it can accept nulls