问题
I am trying to write DBNull.Value using Parameters.AddWithVallue when an optional parameter (a string) of a C# method is null.
public static void Abc(string distrito, string municipio = null)
command.Parameters.AddWithValue("@Municipio", municipio ?? (object)DBNull.Value);
command.Parameters.AddWithValue("@Municipio", municipio.Length > 0 ? municipio : (object)DBNull.Value);
However, (object)DBNULL returns two different values in two different working ways. One writes empty/null and other NULL.
回答1:
I'm going to make a few assumptions, but I think I know what's going on. municipio is probably an empty string. It is not null.
In that case, municipio ?? (object)DBNull.Value will be an empty string, not null. However, municipio.Length > 0 ? municipio : (object)DBNull.Value has a value of DBNull.Value, which will generate a null in SQL Server. In this case, if municipio is null, then this code will throw. Since you say the code runs, I'm assuming that municipio is not null.
来源：https://stackoverflow.com/questions/41473095/dbnull-cast-to-object-returns-different-value