Hi,

Please refer below code

C#

protected void Insert(object sender, EventArgs e)
{
    using (CustomersDataContext ctx = new CustomersDataContext())
    {
        Customer customer = (from c in ctx.Customers
                                where c.CustomerId == txtId.Text;
                                select c).FirstOrDefault();
        if(customer == null)
        {
            Customer customer = new Customer
            {
                Name = txtName.Text,
                Country = txtCountry.Text
            };
            ctx.Customers.InsertOnSubmit(customer);
            ctx.SubmitChanges();
        }else{
                // Display message of already exist
        }
    }
    this.BindGrid();
}

from referred article

Select Insert Edit Update and Delete in GridView using LINQ to SQL in ASP.Net