Sử dụng DropDownList trong Control GridView ASP.NET

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
        BindData();
}
private void BindData()
{
    //Bind the grid view
    gvProducts.DataSource = RetrieveProducts();
    gvProducts.DataBind();
}
private DataSet RetrieveProducts()
{
    if (ViewState["Products"] != null)
        return (DataSet)ViewState["Products"];
    //fetch the connection string from web.config
    string connString =
      ConfigurationManager.ConnectionStrings["Sql"].ConnectionString;
    //SQL statement to fetch entries from products
    string sql = @"Select top 10  P.ProductID, P.Name as ProductName,
                    P.ProductNumber, ListPrice, PS.ProductSubCategoryID,
                    ps.Name as SubCategoryName 
                    from Production.Product P
                    INNER JOIN Production.ProductSubcategory PS
                    ON P.ProductSubcategoryID = PS.ProductSubcategoryID";
    DataSet dsProducts = new DataSet();
    //Open SQL Connection
    using (SqlConnection conn = new SqlConnection(connString))
    {
        conn.Open();
        //Initialize command object
        using (SqlCommand cmd = new SqlCommand(sql, conn))
        {
            SqlDataAdapter adapter = new SqlDataAdapter(cmd);
            //Fill the result set
            adapter.Fill(dsProducts);
        }
    }
    ViewState["Products"] = dsProducts;
    return dsProducts;
}

protected void gvProducts_RowEditing(object sender,
                                        GridViewEditEventArgs e)
{
    gvProducts.EditIndex = e.NewEditIndex;
    BindData();
}
protected void gvProducts_RowDataBound(object sender, GridViewRowEventArgs e)
{
    if (e.Row.RowType == DataControlRowType.DataRow)
    {
        //check if is in edit mode
        if ((e.Row.RowState & DataControlRowState.Edit) > 0)
        {
            DropDownList ddlSubCategories =
                      (DropDownList)e.Row.FindControl("ddlSubCategories");
            //Bind subcategories data to dropdownlist
            ddlSubCategories.DataTextField = "SubCategoryName";
            ddlSubCategories.DataValueField = "ProductSubcategoryID";
            ddlSubCategories.DataSource = RetrieveSubCategories();                   
            ddlSubCategories.DataBind();
            DataRowView dr = e.Row.DataItem as DataRowView;
            ddlSubCategories.SelectedValue =
                         dr["ProductSubCategoryID"].ToString();
        }
    }
}
private DataTable RetrieveSubCategories()
{
   //fetch the connection string from web.config
    string connString =
            ConfigurationManager.ConnectionStrings["Sql"].ConnectionString;
    //SQL statement to fetch entries from products
    string sql = @"Select ProductSubcategoryID, Name as SubCategoryName
                from Production.ProductSubcategory";
    DataTable dtSubCategories = new DataTable();
    //Open SQL Connection
    using (SqlConnection conn = new SqlConnection(connString))
    {
        conn.Open();
        //Initialize command object
        using (SqlCommand cmd = new SqlCommand(sql, conn))
        {
            SqlDataAdapter adapter = new SqlDataAdapter(cmd);
            //Fill the result set
            adapter.Fill(dtSubCategories);
        }
    }
    return dtSubCategories;
}

protected void gvProducts_RowUpdating(Object sender,
                                        GridViewUpdateEventArgs e)
{
    // Get the product id of the selected product   
    string productID = gvProducts.DataKeys[e.RowIndex].Value.ToString();
    // Get the GridViewRow object that represents the row being edited
    // from the Rows collection of the GridView control.           
    GridViewRow row = gvProducts.Rows[e.RowIndex];
 
    // Get the controls that contain the updated values. In this
    // example, the updated values are contained in the TextBox
    // controls declared in the edit item templates of each TemplateField
    // column fields in the GridView control.
    TextBox txtProductNumber = (TextBox)row.FindControl("txtProductNumber");
    TextBox txtProductName = (TextBox)row.FindControl("txtProductName");
    DropDownList ddlSubCategories =
                      (DropDownList)row.FindControl("ddlSubCategories");
    TextBox txtListPrice = (TextBox)row.FindControl("txtListPrice");
    //update the product
    UpdateProduct(productID, txtProductName.Text,
                                ddlSubCategories.SelectedValue,
                                txtListPrice.Text);
    gvProducts.EditIndex = -1;
    ViewState["Products"] = null;
    BindData();
}
Private void UpdateProduct(string productID,
                string productName, string subCategoryID, string listPrice)
{
//fetch the connection string from web.config
string connString =
       ConfigurationManager.ConnectionStrings["Sql"].ConnectionString;
//SQL statement to update a product
string sql = String.Format(@"Update Production.Product set Name='{0}'                                   
                        ,ProductSubcategoryID={1}
                        ,ListPrice ={2}
                        where ProductID = {3}",
                                productName,
                                subCategoryID,
                                listPrice, productID);
using (SqlConnection conn = new SqlConnection(connString))
{
    conn.Open();
    //Initialize command object
    using (SqlCommand cmd = new SqlCommand(sql, conn))
    {
        cmd.ExecuteNonQuery();
    }
}