int? i;
object o;

// this is ok:
if (i.HasValue)
    o = i;
else
    o = DBNull.Value;

// This fails to compile. Error 1 Type of conditional expression cannot be determined
// because there is no implicit conversion between 'int?' and 'System.DBNull'
o = i.HasValue ? i : DBNull.Value;

// these also fail for the same reason
o = i == null ? DBNull.Value : i;
o = i.HasValue ? i : "ABC";

// but these works:
o = i.HasValue ? (object)i : DBNull.Value;
o = i.HasValue ? (object)i : (object)DBNull.Value;
o = i.HasValue ? i : (object)DBNull.Value;