Question

I have two tables, for simplicity lets pretend they’re defined like this

Stack 
{
    ID       int  Primary Key
    Name     varchar(255), not null
}

User
{
    ID           int  Primary Key
    Name         varchar(255), not null
    Stack_ID     int, Foreign Key to Stack.ID nulls are allowed
}

My stored procedure for inserting a User looks something like this

CREATE PROCEDURE usp_insertUser
    @id int,
    @name varchar(255),
    @stackID int
AS
    INSERT INTO User (ID, Name, Stack_ID) VALUES (@id, @name, @stackID)
GO

In my Model layer my user class I use a nullable to hold StackID

public class User
{
     public int ID {get; set;}
     public string Name {get; set;}
     int? Stack_ID {get; set;}
}

My Data Access Layer I thought was pretty standard, but i’m having trouble, my InsertUser method looks something similar to this

 public void InsertUser(User entity)
 {
     ...
     using (SqlCommand command = new SqlCommand())
     {
         command.Connection = conn;
         command.CommandType = System.Data.CommandType.StoredProcedure;
         command.CommandText = "usp_insertUser";

         command.Parameters.AddWithValue("@id", entity.ID);
         command.Parameters.AddWithValue("@name", entity.Name);
         command.Parameters.AddWithValue("@stackID", entity.Stack_ID);
         conn.Open();
         ...
         //real stored proc returns the id of the item inserted so i use a reader
         SqlDataReader reader = command.ExecuteReader();
         ....
    }
 }

My problem here, when I leave User.Stack_ID null, it doesn’t insert a null in the database, instead I get an error, saying that @stackID was expected but wasn’t there. Why can’t I just straight up insert this nullable int into the database?

Also, is there some one line way I could read this possible null int from the database?

When I did this I assumed this would work.

User.Stack_ID = (int?)reader["Stack_ID"];

that obviously doesn’t work, and I have to do something sloppy like

try {
   User.StackID = (int)reader["Stack_ID"];
}
catch {
   User.StackID = null;
}

any help would be appreciated with working with these Nullable type. Sorry for the noob questions, in the past when I had ints and wanted a null value or w/e I would set them to -1 or 0 or something, that doesn’t work when they’re foreign keys 🙁

Answer 1

command.Parameters.AddWithValue("@stackID", entity.Stack_ID ?? (object) DBNull.Value);

To read a null value, you could do this:

User.StackID = reader.Field<int?>("Stack_ID");

This uses the Field extension method. From MSDN:

The Field method provides support for accessing columns as nullable types. If the underlying value in the DataSet is [DBNull.]Value, the returned nullable type will have a value of null.