How to Update all GridView Rows on Button click

Ref: How to bind data to a DropDownList which is inside a GridView in ASP.Net

HTML:

<form id="form1" runat="server">
<div>
    <asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false" OnRowDataBound="OnRowDataBound">
        <Columns>
            <asp:BoundField DataField="Id" HeaderText="Id" />
            <asp:TemplateField HeaderText="Country">
                <ItemTemplate>
                    <asp:Label ID="lblCity" runat="server" Text='<%# Eval("City") %>' Visible="false" />
                    <asp:DropDownList ID="ddlCities" runat="server">
                    </asp:DropDownList>
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>
</div>
<asp:Button Text="Update" OnClick="Update" runat="server" />
</form>

C#:

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        GridView1.DataSource = GetData("SELECT * FROM Cities");
        GridView1.DataBind();
    }
}

private DataSet GetData(string query)
{
    string conString = ConfigurationManager.ConnectionStrings["ConString"].ConnectionString;
    SqlCommand cmd = new SqlCommand(query);
    using (SqlConnection con = new SqlConnection(conString))
    {
        using (SqlDataAdapter sda = new SqlDataAdapter())
        {
            cmd.Connection = con;
            sda.SelectCommand = cmd;
            using (DataSet ds = new DataSet())
            {
                sda.Fill(ds);
                return ds;
            }
        }
    }
}

protected void OnRowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow)
    {
        //Find the DropDownList in the Row
        DropDownList ddlCities = (e.Row.FindControl("ddlCities") as DropDownList);
        ddlCities.DataSource = GetData("SELECT DISTINCT City  FROM Cities");
        ddlCities.DataTextField = "City";
        ddlCities.DataValueField = "City";
        ddlCities.DataBind();

        //Add Default Item in the DropDownList
        ddlCities.Items.Insert(0, new ListItem("Please select"));

        //Select the Country of Customer in DropDownList
        string city = (e.Row.FindControl("lblCity") as Label).Text;
        ddlCities.Items.FindByValue(city).Selected = true;
    }
}

protected void Update(object sender, EventArgs e)
{
    foreach (GridViewRow row in this.GridView1.Rows)
    {
        int id = Convert.ToInt32(row.Cells[0].Text);
        string city = (row.FindControl("ddlCities") as DropDownList).SelectedItem.Value;
        UpdateRecord(id, city);
    }
}

private void UpdateRecord(int id, string city)
{
    string constr = ConfigurationManager.ConnectionStrings["ConString"].ConnectionString;
    string sqlStatment = "UPDATE Cities SET City  = @City WHERE Id = @id";
    using (SqlConnection con = new SqlConnection(constr))
    {
        using (SqlCommand cmd = new SqlCommand(sqlStatment, con))
        {
            con.Open();
            cmd.Parameters.AddWithValue("@Id", id);
            cmd.Parameters.AddWithValue("@city", city); 
            cmd.ExecuteNonQuery();
            con.Close();
        }
    }
}

NameSpace:

using System.Data;
using System.Data.SqlClient;
using System.Configuration;

SQL:

CREATE TABLE [dbo].[Cities](
	[Id] [int] IDENTITY(1,1) NOT NULL,
	[City] [varchar](50) NOT NULL,
 CONSTRAINT [PK_Cities] PRIMARY KEY CLUSTERED 
(
	[Id] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]

GO

Image: