数据库操作问题

SqlDataAdapter ada=new SqlDataAdapter();
SqlCommand com=new SqlCommand("insertRec",new SqlConnection("database=test;uid=sa;pwd=;"));
com.CommandType=CommandType.StoredProcedure;
ada.TableMappings.Add("Table","user"); com.Parameters.Add("@name",SqlDbType.NVarChar,10);
com.Parameters.Add("@age",SqlDbType.NVarChar,10);
                                    SqlParameter p=new SqlParameter("@id",SqlDbType.Int);
p.Direction=ParameterDirection.Output ;
                                     com.Parameters.Add(p);
ds=new DataSet();
table=new DataTable("user");
table.Columns.Add("id",typeof(Int32));
table.Columns.Add("name",typeof(String));
table.Columns.Add("age",typeof(String));
table.Columns["id"].AllowDBNull=false;
table.Columns["id"].AutoIncrement=true;
ds.Tables.Add(table);
DataRow dr=ds.Tables["user"].NewRow();
dr["name"]="wang";
dr["age"]="20";
ds.Tables["user"].Rows.Add(dr);
ada.InsertCommand=com;
ada.InsertCommand.Parameters[0].SourceColumn ="name";
ada.InsertCommand.Parameters[1].SourceColumn ="age";
ada.Update(ds,"user");
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ALTER PROCEDURE insertRec
(
@id int output,
 @name nvarchar(10),@age nvarchar(10)
) 
as
insert test (username,age)
values( @name,@age)
select @id=@@identity
return 0
------------------------------------------
提示无法将null插入到列id
请问怎么改？谢谢