Import Namespace Like,
using System.Data.SqlTypes;
and insert null like (SqlDouble.Null) if field is double and (SqlInt16.Null) if field is int and SqlString.Null if it is varchar in database depending upon datatype.
To add default value to your dataset,You can try this :
DataSet dataset = new DataSet();
dataset.Tables[0].Columns["Column_Name"].DefaultValue = "default_value";