How to Insert Null Values into a DataBase Field

I have a function that updates a table in sql server

If a field is left blank I want the database field set to Null. I use:
sqlCmd.Parameter.Add("@.somefield, someintvalue) to pass all my values to the database.

if someintvalue is null I want to set @.somefield to dbnull

How can I do this? I get errors if I submit a null integer when there is a foreign key constraint. What is the best way to handle this? should I just make all parameters objects? so that I can set them to DBNull.Value?

Also on a side note, when you are populating fields from a datatable, is there a better way to set the values (i.e. of a textbox) than cheking for DBNull before assigning?

check this out|||

FOREIGN KEY can allow NULL values if it is a UNIQUE constraint. Run a search in the BOL(books online) enable NULL on FOREIGN KEY. The following is from the BOL. Hope this helps.

"A FOREIGN KEY constraint does not have to be linked only to a PRIMARY KEY constraint in another table; it can also be defined to reference the columns of a UNIQUE constraint in another table. A FOREIGN KEY constraint can containnull values; however, if any column of a composite FOREIGN KEY constraint containsnull values, then verification of the FOREIGN KEY constraint will be skipped."

sql