u can directly apply validation at property 

using System.windows.forms;

set
{
if(value.length==0)
{
messagebox.show("error message")
}
variablenmame=value;
}

can anybody give me examples for black box testing and white 
box testing

on the  button's click event use response.redirect("Name of 
the file with extension");

x=x-a;
a=x+a;
x=a-x;

x=x-p;
p=x+p;
x=p-x;

session["name of the session"]=values;
response.write session["Name"];

values means the data you want to store in the session like 
session["uid"]=ds.tables[0].rows[0]["uid"].tostring();