Assign null to a SqlParameter

The problem is that the ?: operator cannot determine the return type because you are either returning an int value or a DBNull type value, which are not compatible.

You can of course cast the instance of AgeIndex to be type object which would satisfy the ?: requirement.

You can use the ?? null-coalescing operator as follows

SqlParameter[] parameters = new SqlParameter[1];     
SqlParameter planIndexParameter = new SqlParameter("@AgeIndex", SqlDbType.Int);
planIndexParameter.Value = (object)AgeItem.AgeIndex ?? DBNull.Value;
parameters[0] = planIndexParameter; 

Here is a quote from the MSDN documentation for the ?: operator that explains the problem

Either the type of first_expression and second_expression must be the same, or an implicit conversion must exist from one type to the other.

The accepted answer suggests making use of a cast. However, most of the SQL types have a special Null field which can be used to avoid this cast.

For example, SqlInt32.Null "Represents a DBNull that can be assigned to this instance of the SqlInt32 class."

int? example = null;
object exampleCast = (object) example ?? DBNull.Value;
object exampleNoCast = example ?? SqlInt32.Null;