Pass Context Key to AJAX AutoCompleteExtender in GridView / DataList in ASP.Net

You need to use DataList OnItemDataBound Event to attach the TargetControlId and DropDownList OnSelectedIndex change event to Set the Context key.

HTML

<div>
    <asp:ScriptManager ID="ScriptManager1" runat="server" EnablePageMethods="true">
    </asp:ScriptManager>
    <asp:DataList ID="dlItems" runat="server" OnItemDataBound="OnItemDataBound">
        <ItemTemplate>
            <asp:DropDownList ID="ddlCountries" runat="server" OnSelectedIndexChanged="OnSelectedIndexChanged"
                AutoPostBack="true">
                <asp:ListItem Text="All Countries" Value="0"></asp:ListItem>
                <asp:ListItem Text="Argentina" Value="Argentina"></asp:ListItem>
                <asp:ListItem Text="Austria" Value="Austria"></asp:ListItem>
                <asp:ListItem Text="Belgium" Value="Belgium"></asp:ListItem>
                <asp:ListItem Text="Brazil" Value="Brazil"></asp:ListItem>
                <asp:ListItem Text="Canada" Value="Canada"></asp:ListItem>
                <asp:ListItem Text="Denmark" Value="Denmark"></asp:ListItem>
                <asp:ListItem Text="Finland" Value="Finland"></asp:ListItem>
                <asp:ListItem Text="France" Value="France"></asp:ListItem>
                <asp:ListItem Text="Germany" Value="Germany"></asp:ListItem>
                <asp:ListItem Text="Ireland" Value="Ireland"></asp:ListItem>
                <asp:ListItem Text="Italy" Value="Italy"></asp:ListItem>
                <asp:ListItem Text="Mexico" Value="Mexico"></asp:ListItem>
                <asp:ListItem Text="Norway" Value="Norway"></asp:ListItem>
                <asp:ListItem Text="Poland" Value="Poland"></asp:ListItem>
                <asp:ListItem Text="Portugal" Value="Portugal"></asp:ListItem>
                <asp:ListItem Text="Spain" Value="Spain"></asp:ListItem>
                <asp:ListItem Text="Sweden" Value="Sweden"></asp:ListItem>
                <asp:ListItem Text="Switzerland" Value="Switzerland"></asp:ListItem>
                <asp:ListItem Text="UK" Value="UK"></asp:ListItem>
                <asp:ListItem Text="USA" Value="USA"></asp:ListItem>
                <asp:ListItem Text="Venezuela" Value="Venezuela"></asp:ListItem>
            </asp:DropDownList>
            <asp:TextBox ID="txtContactsSearch" runat="server"></asp:TextBox>
            <cc1:AutoCompleteExtender ServiceMethod="SearchCustomers" MinimumPrefixLength="2"
                CompletionInterval="100" EnableCaching="false" CompletionSetCount="10" TargetControlID="txtContactsSearch"
                UseContextKey="true" ID="AutoCompleteExtender1" runat="server" FirstRowSelected="false">
            </cc1:AutoCompleteExtender>
        </ItemTemplate>
    </asp:DataList>
    <asp:Button Text="Save" runat="server" />
</div>

Namespaces

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

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        DataTable dt = new DataTable();
        dt.Columns.Add("Id");
        dt.Rows.Add();
        this.dlItems.DataSource = dt;
        this.dlItems.DataBind();
    }
}

protected void OnSelectedIndexChanged(object sender, EventArgs e)
{
    DataListItem item = (sender as DropDownList).NamingContainer as DataListItem;
    AutoCompleteExtender autoCompleteExtender = item.FindControl("AutoCompleteExtender1") as AutoCompleteExtender;
    autoCompleteExtender.ContextKey = (sender as DropDownList).SelectedItem.Value;
}

protected void OnItemDataBound(object sender, DataListItemEventArgs e)
{
    if (e.Item.ItemType == ListItemType.Item || e.Item.ItemType == ListItemType.AlternatingItem)
    {
        TextBox txt = e.Item.FindControl("txtContactsSearch") as TextBox;
        DropDownList ddl = e.Item.FindControl("ddlCountries") as DropDownList;
        AutoCompleteExtender autoCompleteExtender = e.Item.FindControl("AutoCompleteExtender1") as AutoCompleteExtender;
        autoCompleteExtender.ContextKey = ddl.SelectedItem.Value;
        autoCompleteExtender.TargetControlID = txt.ID;
        autoCompleteExtender.ServiceMethod = "SearchCustomers";
        autoCompleteExtender.UseContextKey = true;
    }
}

[System.Web.Script.Services.ScriptMethod()]
[System.Web.Services.WebMethod]
public static List<string> SearchCustomers(string prefixText, int count, string contextKey)
{
    using (SqlConnection conn = new SqlConnection())
    {
        conn.ConnectionString = ConfigurationManager
                .ConnectionStrings["constr"].ConnectionString;
        using (SqlCommand cmd = new SqlCommand())
        {
            string cmdText = "select ContactName from Customers where " +
            "ContactName like @SearchText + '%'";
            cmd.Parameters.AddWithValue("@SearchText", prefixText);
            if (contextKey != "0")
            {
                cmdText += " and Country = @Country";
                cmd.Parameters.AddWithValue("@Country", contextKey);
            }
            cmd.CommandText = cmdText;
            cmd.Connection = conn;
            conn.Open();
            List<string> customers = new List<string>();
            using (SqlDataReader sdr = cmd.ExecuteReader())
            {
                while (sdr.Read())
                {
                    customers.Add(sdr["ContactName"].ToString());
                }
            }
            conn.Close();
            return customers;
        }
    }
}

Screenshot