string[] names = { "Akash", "Ganga", "Sukaoi" };
ServerValidateEventArgs svea = new ServerValidateEventArgs("names", true);
    protected void Button1_Click(object sender, EventArgs e)
    {
        ValidateName(TextBox1.Text,svea);

    }

public void ValidateName(object sender, ServerValidateEventArgs e)
    {
        foreach (string a in names)
        {
            if (e.Value.Equals(names))
            {
                Response.Write("Name already exists");
            }
            else
            {
               e.IsValid = false;
            }
        }
    }