尝试将null分配给可为null的int会导致“在'int'和'null'之间没有隐式转换”错误

Question

public List<Tuple<int, int, int, string, int, int, Tuple<bool, bool, int?>>> 
    GetValues(int someId, int someOtherlId)
{
    var t = new List<Tuple<int, int, int, string, int, int, Tuple<bool, bool, int?>>>();
    using (var context = new Entities(ConnString))
    {
        var pc = context.SomeTable.Where(c => c.SomeId == someId && 
            c.SomeOtherId == someOtherId).OrderBy(c => c.Id).ToList();
        t.AddRange(pc.Select(cols => new Tuple<int, int, int, string, int, int, Tuple<bool, bool, int?>>
            (cols.Id, cols.someId, cols.someOtherId, cols.ColumnName, cols.MinDataLength,
            cols.MaxDataLength, new Tuple<bool, bool, int?>(cols.Required, cols.Eav.HasValue && cols.Eav.Value,
                cols.AttribId.HasValue ? cols.AttribId.Value : null))));
    }
        return t;
    }
}

Can't figure out why this code won't compile and generates "There is no implicit conversion between 'int' and 'null'" error.

In that second Tuple the int? is consistently marked as nullable so why is it being detected as a normal int by the compiler in t.AddRange which generates "Type of conditional expression cannot be determined because there is no implicit conversion between 'int' and ''" error at cols.AttribId.HasValue ? cols.AttribId.Value : null cols.AttribId.HasValue ? cols.AttribId.Value : null .

Any ideas?

Answer 1

Try this:

cols.AttribId.HasValue ? (int?)cols.AttribId.Value : null

What's happening is that the C# ternary there has to exist implicit conversion between both values. When you write:

cols.AttribId.HasValue ? cols.AttribId.Value : null

cols.AttribId.Value is an int which cannot be converted to null . An int? however can be converted to null .

Answer 2

If you're trying to add an int? to a tuple that already accepts int? why not just add it directly to your tuple—ie replace:

new Tuple<bool, bool, int?>(..., cols.AttribId.HasValue ? cols.AttribId.Value : null)

With

new Tuple<bool, bool, int?>(..., cols.AttribId)

Or to a little bit cleaner, take advantage of type inference with Tuple.Create :

Tuple.Create(..., cols.AttribId)