how to avoid to insert same data into database using C#

[GAURAV MANUSMAR]

There is one database name as "email_book " and in that database there is one table named as "contact_book". ...name and mail_id are the two column which i had been created in that table.......now i want to add data into to my table using form in C# but on the counter part i just want to avoid inserting the repeated data into the table.......whe n user add same mail_id which is already present in the table then it must gives the message that "this mail_address is already present in database"...... ..so for that i need coading in C#

.....please help its urgent

[audrey45]

if you are using EF
DatabaseName db = new DatabaseName();

var checkMail = db.TableName.Si ngle(d => d.mail_id == userMailId);
if(checkMail == null){
//Save your email
}
else{
//Message: email is already exist
}