When working with ADO.NET, without a table adapter, to set a
row's value to null, use the DBNull.Value construct. This can also work with
command objects, when setting up the parameter. If you want to set a parameter
to null, simply assign DBNull.Value to the command's parameter definition, as
shown below:
Listing 7
Comand.Parameters.Add("@Field", SqlDbType.Varchar).Value = DBNull.Value
This translates to a null value in the database.