C# 出口don'；不要插入其他插入

我已经开始学习asp.net。我正在使用VS 2013 Express for C#。 如何使那个些如果情况下检查一个重复的值，若这个值是存在的，那个么我会得到一个关于它的红色摘要，不能插入到DB else插入到数据库和更新按钮

你能帮忙吗

SqlConnection con = new SqlConnection(@"Data Source=TSS\SQLEXPRESS;Initial Catalog=DB;Integrated Security=True");

protected void Add(object sender, EventArgs e)
{
    var vardas = GridView1.FooterRow.FindControl("txtname") as TextBox;
    var pavarde = GridView1.FooterRow.FindControl("txtlastname") as TextBox;
    var pozymis = GridView1.FooterRow.FindControl("DropDownList2") as DropDownList;

    SqlCommand comm = new SqlCommand();
    comm.CommandText = "insert into asmenys (name,lastname, status) values(@name,@lastname, @status)";
    comm.Connection = con;

    comm.Parameters.AddWithValue("@name", name.Text);
    comm.Parameters.AddWithValue("@lastname", lastname.Text);
    comm.Parameters.AddWithValue("@status", status![enter image description here][1].Text);

    con.Open();
    comm.ExecuteNonQuery();
    con.Close();

    DataBind();
}

---

当您说检查值是否存在时，哪些字段不应该有重复项？这些字段必须先编写select语句来检查它们是否存在

范例

protected void Add(object sender, EventArgs e)
{
    var vardas = GridView1.FooterRow.FindControl("txtname") as TextBox;
    var pavarde = GridView1.FooterRow.FindControl("txtlastname") as TextBox;
    var pozymis = GridView1.FooterRow.FindControl("DropDownList2") as DropDownList;

    SqlCommand comm = new SqlCommand();

    comm.CommandText = "select lastname from asmenys where lastname = @lastname";
    comm.Parameters.AddWithValue("@lastname", lastname.Text);

    SqlDataReader reader = comm.ExecuteReader();

    if (reader.HasRows)
    {
        Console.WriteLine("Values exist");
    }
    else
    {
        comm.CommandText = "insert into asmenys (name,lastname, status) values(@name,@lastname, @status)";
        comm.Connection = con;

        comm.Parameters.AddWithValue("@name", name.Text);
        comm.Parameters.AddWithValue("@lastname", lastname.Text);
        comm.Parameters.AddWithValue("@status", status![enter image description here][1].Text);

        con.Open();
        comm.ExecuteNonQuery();
        con.Close();

        DataBind();
    }
}