C# ?? null coalescing operator question

Question

I have defined Class Person property Birthday as nullable DateTime? , so why shouldn’t the null coalescing operator work in the following example?

cmd.Parameters.Add(new SqlParameter("@Birthday",
   SqlDbType.SmallDateTime)).Value =       
     person.Birthday ?? DBNull.Value; 

The compiler err I got was "Operator '??' cannot be applied to operands of type 'System.DateTime?' and 'System.DBNull'"

The following also got a compile error :

cmd.Parameters.Add(new SqlParameter("@Birthday", 
  SqlDbType.SmallDateTime)).Value = 
   (person.Birthday == null) ? person.Birthday:DBNull.Value;

I added a cast to (object) as recommended by Refactor, and it compiled, but didn’t work properly and the value was stored in the sqlserver db as null in both cases.

SqlDbType.SmallDateTime)).Value =       
         person.Birthday ?? (object)DBNull.Value;

Can someone explain what is going on here?

I needed to use the following clumsy code:

   if (person.Birthday == null) 
    cmd.Parameters.Add("@Birthday", SqlDbType.SmallDateTime).Value 
      = DBNull.Value;
     else cmd.Parameters.Add("@Birthday", SqlDbType.SmallDateTime).Value = 
          person.Birthday;

Answer 1

The problem is that DateTime? and DBNull.Value are not the same type so you can't use the null coalescing operator on them.

In your case you can do person.Birthday ?? (object)DBNull.Value to pass a value of type object through to Add()